# 37 MPG SOUNDS RIGHT TO YOU? OR I'M PARANOID?



## andycorleone (27 Nov 2007)

Hi, 37 mpg sounds right to you for an small automatic car? ( distanced covered 260 miles liters of fuel used 35) or is something wrong in it? (BTW I just changed the Sparks just in case that was the problem)


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## MrJay (27 Nov 2007)

What were you expecting? Quoted mpgs are attainable only with an empty car running at a continuous known speeds on rollers in the lab. Put a driver in the car, chuck some stuff in the boot, fill the tank up, start and stop at a few traffic lights, go up a hill, deflate the tyres a little and drive at anything other than optimal speed then can forget getting anything like 45mpg.

I'd guess that 37mpg is actually rather good.

In the US the average mpg in 2005 was 19.8mpg. The Ford Model T managed 21mpg. Go figure.


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## motownmartin (27 Nov 2007)

Don't listen to other blokes bragging about how many mpg they get in their car, someone else's car is always better than yours, they also go faster :roll: 

Sometimes mine will do 45mpg and other times 35mpg, depends on how fast and how far.


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## Matt_S (27 Nov 2007)

what size engine, and is it petrol? I'm sure your 37mpg is corrrect for the car, isn't great figures for a small car though is it!!! As mentioned earlier we've really not progressed much is this area of technology have we!


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## WellsWood (27 Nov 2007)

Hmmph!!! Try driving a 15 yr old 2.0l auto Volvo estate - you'll have wet dreams about 37mpg. :roll: 
[-(


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## andycktm (27 Nov 2007)

we've got a 1.6 petrol focus which does 34-36mpg and a peugeot 307 diesel
which does 58-60mpg its also quicker than the focus.


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## Woody Alan (27 Nov 2007)

> Sometimes mine will do 45mpg and other times 35mpg, depends on how fast and how far.


Hmm sometimes mine does 25mpg never less and maybe 30 mpg it's funny how it does more when the wife drives...mind you she's about 5 stone lighter so I reckon that's it.


> Don't listen to other blokes bragging


My mate has a 1.8 mercedes auto and insists he gets 40 mpg I've always suspected he's making it up but then again he lives in the sticks so the only driving he does is on a run, not suburban.

Alan


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## Paul Chapman (27 Nov 2007)

Sounds quite reasonable to me. Generally an automatic will use more fuel than a manual, given similar conditions and driving (and your brake pads will wear out faster as well :wink: )

Cheers :wink: 

Paul


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## andycorleone (27 Nov 2007)

Thanks for the responces is a C3, I was a bit concern because the technical data says 70 mpg average but maybe this is in a real world usage now I'm more calm about the topic


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## special bone (27 Nov 2007)

70mpg sounds like a diesel figure, 35mpg sounds like a petrol figure.

1987 Saab 900 Turbo. I'd struggle to get 35mpg, even if I turned it off when I went down hill!

Rich

P.S. When I first got it, I got it down to 10mpg  
:twisted: :twisted: :twisted: :twisted: :twisted:


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## Woody Alan (27 Nov 2007)

> P.S. When I first got it, I got it down to 10mpg



That's the trouble with those shopping trips with the missus, you go so slow barely out of 2nd gear. 

I had a ride in a mates dad's Saab 9000 ...bear in mind this was about 20 years ago, we were approaching a dual carriageway and he said I'll show you what it can do... I assumed he meant when we reached the dual not before it, By god I wondered what was happening apparently at the time the Saab could out accelerate a Ferrari (don't know which one) at 50-70 and I believe it ...it was fast. Nowadays I am very impressed by Escudos Audi A8 which has such effortless power, but can still scare the driver whilst the passenger has no idea there is mild panic ensuing 

Alan


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## Waka (27 Nov 2007)

Woody Alan":171v82t9 said:


> > [My mate has a 1.8 mercedes auto and insists he gets 40 mpg I've always suspected he's making it up but then again he lives in the sticks so the only driving he does is on a run, not suburban.
> >
> > Alan



Sounds about right to me, on a long run I get about that from my Merc, mind you around town I'm reduced to about 26 mpg.


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## motownmartin (27 Nov 2007)

Woody Alan":45nngvkh said:


> Hmm sometimes mine does 25mpg never less and maybe 30 mpg it's funny how it does more when the wife drives...mind you she's about 5 stone lighter so I reckon that's it.
> 
> Alan


More likely to be the size of your feet Alan :lol:


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## mailee (27 Nov 2007)

you should be so lucky Mark W, Try driving a 10 year old T5 getting around 22mpg if you drive like a Granny. usually it is more like 18mpg. :lol:


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## Fecn (27 Nov 2007)

I've never had a car which does more than 30mpg.

I manage to scrape 30mpg most of the time in my 2-tonne 10-year-old Toyota HiAce van.. but then I drive like a nun most of the time.

My last car (2-seater Merc) would get down to about 7mpg when I put my foot down... Niiiice


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## motownmartin (27 Nov 2007)

Fecn":3crmpusm said:


> My last car (2-seater Merc) would get down to about 7mpg when I put my foot down... Niiiice


Blimey, it would be cheaper to get a taxi :lol:


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## Fecn (27 Nov 2007)

motownmartin":29zk9os0 said:


> Fecn":29zk9os0 said:
> 
> 
> > My last car (2-seater Merc) would get down to about 7mpg when I put my foot down... Niiiice
> ...



Yeah.. but nothing like as much fun


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## MrJay (27 Nov 2007)

Hang on. Until very recently I had a car. I used it to sit in traffic jams. Fun isn't the first word... Perhaps you don't have traffic jams in where you come from?


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## WellsWood (27 Nov 2007)

mailee":22mhi1r5 said:


> you should be so lucky Mark W, Try driving a 10 year old T5 getting around 22mpg if you drive like a Granny. usually it is more like 18mpg. :lol:



....about the same as my old 240 - going to see the in-laws in that was damned expensive (400 mile round trip). Comfy though :wink:


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## Fecn (28 Nov 2007)

MrJay":h7shut70 said:


> Hang on. Until very recently I had a car. I used it to sit in traffic jams. Fun isn't the first word... Perhaps you don't have traffic jams in where you come from?



Not all the time.

btw.. 7mpg isn't what I'd get on a tank... 7mpg is what the computer thingy said I was getting for very short stretches. I actually drive in a very saintly manner pretty much all the time. Totally clean driving lisence and never been snapped by a speed camera.


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## cambournepete (28 Nov 2007)

I regularly get 36 or so mpg in my mazda premacy - a mid-sized mpv.
On a long trip it goes to over 40.
It hardly matters how I drive it, or whether I'm towing my trailer.


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## motownmartin (28 Nov 2007)

Fecn":1j5womq3 said:


> Totally clean driving lisence and never been snapped by a speed camera.



Ooooops, me thinks you might regret saying that, don't tempt fate and all that :lol: :lol: :lol:


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## woodbloke (28 Nov 2007)

In the Landy (2.5l diesel) I can get 32mpg with all our camping gear and cycles thrown on the back...this is at a steady 55mph on the motorstrasse tho'. Drops a bit at higher speeds...even worse in town  - Rob


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## Paul Chapman (28 Nov 2007)

Of course there are factors other than engine size/type and driving styles that significantly affect fuel consumption. The fitting of catalytic converters has increased fuel consumption and use of air conditioning increases consumption *significantly*. Far cheaper to just open the window.........

Cheers :wink: 

Paul


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## RogerS (28 Nov 2007)

Mazda RX-8 - rotary engine - impossible to get better than 20 mpg ...on any sort of run. Rapidly gets worse the heavier the right foot.

Compare/contrast with the faithful Hyundai Santa Fe turbo-diesel. Quoted mpg of my model is 38mpg and I reckon to get 36 - which is pretty good going, I reckon. Mind you, I only pootle. :wink:


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## StevieB (28 Nov 2007)

> use of air conditioning increases consumption significantly. Far cheaper to just open the window.........



Actually I am not sure thats at all true. Anyone watch Mythbusters on the Discovery channel? Great programme. They showed there was very little difference between aircon and an open window due to the increased drag from the open window. Test was done in a controlled environement at constant speed for a set period of time. Was done on American cars though.

Steve.


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## mr (28 Nov 2007)

Having the air con on in my car has a significant and noticeable (on trips of a couple hundred miles or more) effect on consumption.

Cheers Mike


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## Paul Chapman (28 Nov 2007)

On the basis of (very un-scientific) tests I have done, I would estimate that the use of air conditioning increases fuel consumption by at least 10 mpg. Other members of my family have noticed similar increases in fuel consumption in their cars when the air conditioning is switched on. The effect is so significant that none of us now uses the air conditioning.

The are probably two issues. First, the air conditioning takes power from the engine, which uses more fuel. If the particular car has a relatively small engine, this use of power to drive the air conditioning will result in a noticeable loss in performance, so there will be a tendency to drive the car harder which, again, will use more fuel.

Cheers :wink: 

Paul


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## White House Workshop (28 Nov 2007)

Paul Chapman":16hepizv said:


> Sounds quite reasonable to me. Generally an automatic will use more fuel than a manual, given similar conditions and driving (and your brake pads will wear out faster as well :wink: )
> 
> Cheers :wink:
> 
> Paul


That used to be the case years ago but the fuel differential between automatics and manuals these days is generally within 5%, and that much can disappear if you're stuck in traffic for 30 minutes!

My wife has a small automatic - 1.4 litre engine, not overly powerful, and she averages about 35. She got close to 40 on a run once, but that was unusual. I have a large BMW automatic and on a run I get around 35, and once got close to 40 but that was on an almost empty road with cruise control on and sitting at 65. On the other hand, 20-23 is more like it in heavy town traffic...

On the other hand, the _worst_ consumption I ever got on my bike was 42 mpg, and that was over 120 miles averaging about 95 mph (hit 130 at one point). I only stopped then to get fuel because the low fuel light came on! It normally gets abut 50, but goes up to 55 on a steady run. Much more fun than sitting in a tin box on wheels - and just as comfortable.


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## Slim (28 Nov 2007)

White House Workshop":19h9f0bl said:


> averaging about 95 mph (hit 130 at one point)



Do you want a medal? You should be banned for life for going that speed on a public road! 

I very nearly killed a biker because he was travelling so fast. I wanted to pull out to overtake in the fast lane of a motorway, so I checked my wing mirror and saw a motorbaike a long way behind. I put my indicator on and luckily before I pulled out I checked my wing mirror again. The bike was almost up alongs side me. If I had pulled out, he would have been dead. He must have been doing at least 130mph. I would have been prosecuted and it would have been on my consience for the rest of my life. My life would have been ruined because of a selfish . on a motorbike.


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## Taffy Turner (28 Nov 2007)

mailee":35o6pngd said:


> you should be so lucky Mark W, Try driving a 10 year old T5 getting around 22mpg if you drive like a Granny. usually it is more like 18mpg. :lol:



Yes - I had one of those briefly - it went like the proverbial of a shiny shovel, but it's average speed was rubbish on a long run, as you had to keep stopping to fill it up with petrol!!!!  

Fortunately it was a company car that I was given while mine was on order, so I gave it back after a couple of months - I missed the performance but not the bills!

My BWM M3 did 25 to the gallon, and that made the T5 seem like a Reliant Robin in the performance stakes!

Taffy


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## woodbloke (28 Nov 2007)

Paul Chapman":se3gh03s said:


> On the basis of (very un-scientific) tests I have done, I would estimate that the use of air conditioning increases fuel consumption by at least 10 mpg. Other members of my family have noticed similar increases in fuel consumption in their cars when the air conditioning is switched on. The effect is so significant that none of us now uses the air conditioning.
> 
> The are probably two issues. First, the air conditioning takes power from the engine, which uses more fuel. If the particular car has a relatively small engine, this use of power to drive the air conditioning will result in a noticeable loss in performance, so there will be a tendency to drive the car harder which, again, will use more fuel.
> 
> ...



Paul - doesn't seem to make much difference to fuel economy in the Landy as it's got the aerodynamics of a brick and the performance to match :lol: - Rob


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## White House Workshop (28 Nov 2007)

Slim":1od59ynw said:


> White House Workshop":1od59ynw said:
> 
> 
> > averaging about 95 mph (hit 130 at one point)
> ...


Why should I be ashamed of doing that speed where it is perfectly legal to do so? Did I say where I was? No I didn't, and you jumped to an incorrect conclusion. So yes I'll have the medal please for successfully negotiating the high speed autobahn traffic in Germany - where I was actually one of the slower moving vehicles for most of the time. 

I agree there are some selfish prats around, on bikes and behind wheels, and many behind wheels seem to think that just because they've put their indicator on it gives them a god-given right to pull out. Good job you checked, otherwise a biker's family would have had you on their consciences for the rest of their lives too. It works both ways.

Please don't make assumptions when you aren't in possession of the full facts.


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## Paul Chapman (28 Nov 2007)

woodbloke":28vfh5j1 said:


> Paul - doesn't seem to make much difference to fuel economy in the Landy as it's got the aerodynamics of a brick and the performance to match :lol: - Rob



Yes, but it's cool 8) 8) 8) 

Cheers :wink: 

Paul


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## Slim (28 Nov 2007)

White House Workshop":akljekgf said:


> Slim":akljekgf said:
> 
> 
> > White House Workshop":akljekgf said:
> ...



OK then, I apologise. However, do you not think it would have been sensible for you to mention that you were on an autobahn at the time. :roll: A statement like that is just asking for a response.

I ride as well, and I know full well that travelling at those speeds, whether legal or not, you will not last long. People will pull out on you as to them it will appear safe for them to do so. They will not assume that you are travelling so fast. I checked my wing mirror, and it was clear, so I initiated the pulling out manouver. I did not prepare for the fact that the motorbike was going 60-70mph faster than I was and that he would cover so much distance between me checking my wing mirror and pulling out. Harldy my fault is it?

I and most other drivers do not believe we have a god given right to pull out just because we indicate. If a bike is going so fast that he catches up someone pulling out the it is quite clearly their fault.

Sorry Andy, I didn't mean to hijack your thread.


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## RogerS (28 Nov 2007)

Going slightly OT but still within the motoring area, having switched to driving the 4x4 a lot more than the Mazda, I've experienced a marked difference in other motorists' behaviour.

Previously when driving the Mazda RX8, not stupidly fast but fast-ish (50 - 60 mph) and within the prevailing road and weather conditions etc, I've noticed that very few people will pull out in front you.

Having switched to the 4x4, I tend to drive maybe 10-15mph slower and the nett result is that I spend far more time either making emergency stops or serious deceleration because the dozy git that's just pulled out can't seem to find the accelerator peddle. 

Now I'm not sure whether it is because my speed is slower or it is because people perceive the cars differently due to the difference in body shape etc.

I need to experiment a bit more - change the speeds around between the two cars - but just wondered if any other people had noticed a similar difference in behaviour.


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## Fecn (28 Nov 2007)

Roger Sinden":20omkmj8 said:


> Now I'm not sure whether it is because my speed is slower or it is because people perceive the cars differently due to the difference in body shape etc.
> 
> I need to experiment a bit more - change the speeds around between the two cars - but just wondered if any other people had noticed a similar difference in behaviour.



I noticed the same thing changing from a 2-seater merc to a 2-tonne white van... Now everybody seems to think I'm invisible and cuts me up at every opportunity... The speed i drive at is exactly the same as it always was.


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## White House Workshop (28 Nov 2007)

Slim":3vr3k0nd said:


> [
> OK then, I apologise. However, do you not think it would have been sensible for you to mention that you were on an autobahn at the time. :roll: A statement like that is just asking for a response.


Fair comment - sorry. I thought the thread was about mpg and that was my focus, which was to show that even at high average speeds bikes can have good economy. I shudder to think what my car would do at those sorts of speeds. :?


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## Slim (28 Nov 2007)

White House Workshop":2mvybap7 said:


> Fair comment - sorry. I thought the thread was about mpg and that was my focus, which was to show that even at high average speeds bikes can have good economy. I shudder to think what my car would do at those sorts of speeds. :?



I used to have a 125cc vespa which had a 6 litre tank and would do about 140 miles to a tank. That works out about 105mpg. God I miss it!


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## Woody Alan (28 Nov 2007)

Paul wrote:-


> On the basis of (very un-scientific) tests I have done, I would estimate that the use of air conditioning increases fuel consumption by at least 10 mpg. Other members of my family have noticed similar increases in fuel consumption in their cars when the air conditioning is switched on. The effect is so significant that none of us now uses the air conditioning.


I find that quite astounding as I have kept "accurate!" records for my car for the last six years and can say that the air con does have an effect, probably 1-2 mpg. but not to the degree you suggest, (certainly not enough to be uncomfortable in preference to miserly  and risk the air con going faulty by not keeping the seals working)
What I can see from my table is that winter has a much more pronounced effect 2-4 mpg less in winter with aircon off, but most of my trips are local and short so it's all guzzled in warm up time, wish I had a choke that I could push in once rolling and let out at the lights. If (and I imagine not) anyone is wondering why I keep the records it's because it's an easy way to check whether the car is ailing in some way. In six yeays my Volvo v70 has a total average of 25 mpg mostly all local driving. That's accurate not these computer trip doodahs.

Alan


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## PowerTool (28 Nov 2007)

I've got an 1100 Fiesta (stop laughing..) and average 41mpg,doing the 12 miles each way to work (mainly dual carriageway),and round town on a weekend.Can get about 46mpg out of it on a 70mph motorway run.

It's not exciting,though  

Andrew


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## andycktm (28 Nov 2007)

When we bought our last car we went for a test run in a focus,
at the the garage the wife liked the look of a certain peugeot 307 1.6 HDI. 
So we had a test run in it and i could not stop grinning to myself,
it reminded me of the RD350lc two stoke bike i used to have ,(well not quite)
its a flyer(120mph err so i've been told :roll: ) and now 1 year later i'm still grinning high 50s mpg.


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## exigetastic (29 Nov 2007)

SWMBO's Nissan Navara is showing 36.5 MPG on the computer :shock: (average over the 10k miles we've owned it) 

Complete overkill for the school run, but it's very handy.

Only downside is the size of the tank, £80 last time I filled it. (Funny it always seems she times so when I need it the lights on)

My car is probably only getting high 20s at a guess, but then I don't keep records, just fill it up when the light goes on  My car is much more fun though :wink: 

Si


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## Streepips (29 Nov 2007)

Been thinking about the earlier statement about it being cheaper to open a window than to run the air con. Read through posts since then, might have missed it but seen no comment to contradict it, so here's mine:
Now bearing in mind that it is just not on to mention going over the speed limit, I will restrict my example to one that safely stays within the motorway speed limit.
Example is:

Fully loaded van say a Merc sprinter, tranny Vivaro, that sort of size, with full payload, in very light traffic ( say 3:30AM ) and an uphill gradient.
You are slowly gaining on a much larger vehicle, you are going say 5 to 10 MPH faster than the vehicle in front but no way are you going to overtake like a 20 yr old Saab Ferrari eater.........you get as close as sensible and safe then indicate and pull out. yes there is a drag effect from the aerodynamics, but you are coming alongside and proceeding to get past..
Now open a window, any window but lets say in this instance the driver side window so as not to compromise the airflows more than necessary.....and watch the vehicle on the left either lock speed with you or draw ahead again.....
I doubt that turning on the aircon instead of opening the window would have such a drastic drag effect....Therefore the engine efficiency and fuel consumption would seem to be more adversely effected by opening the window than by operating the air conditioning.... 

Just in case anyone is thinking that there are a lot more metres cubed in a large van as opposed to a car, and acting as a parachute, this example is based on a full bulkhead type,closed of from the rear of the front seats so probably less overall cubic space than all but the smallest car

Plus it blows the froth off your pint all over the Times crossword....


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## Paul Chapman (29 Nov 2007)

If you watch your rev counter when you turn the air conditioning on and off, it will give you a good indication of how much power it is taking from the engine. Quite a lot in my experience.

Cheers :wink: 

Paul


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## White House Workshop (30 Nov 2007)

Fortunately the a/c doesn't work in my car - there's a slow leak and it's not worth having it fixed for the 2 or 3 days I might need it each year, and then I'll ride my bike anyway! However, my Bentley used a LOT of extra fuel to power the a/c and took the round-town average down by about 20%. Never tried it on a run so couldn't comment, but the compressor for that car was MASSIVE so I'm not surprised it took a fair bit of power to run it. The air temp coming out of the vents was below freezing on max power!

Incidentally - I used to get 20 mpg on a run, 15-16 around town, and was lucky to get 12 around town with the a/c going. Actually not bad for a car with a 7 litre 400 bhp engine and weighing in at almost 3 tons! Wish I still had it sometimes as it was so comfortable and so much fun to drive.


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## RogerM (30 Nov 2007)

Streepips":28hvgn8r said:


> Been thinking about the earlier statement about it being cheaper to open a window than to run the air con. Read through posts since then, might have missed it but seen no comment to contradict it, so here's mine:
> Now bearing in mind that it is just not on to mention going over the speed limit, I will restrict my example to one that safely stays within the motorway speed limit.
> Example is:
> 
> ...



I'd agree with this - and so does "Honest John" in the Saturday Telegraph - a/c is more economical than an open window at speed. I have a 2.0 VW Golf TDi (the 140 bhp) and over the last 4000 miles it's returned an average of 52.3 mpg, and if I'm really careful can get close to 60 mpg on a run. Cruising at 75mph on the M way returns 54 mpg. Much of the time I have the climate control on, and I do not notice any difference between when it is on or off. Maybe A/C is getting more efficient?


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## Lee Brubaker (1 Dec 2007)

After a quick scan of the topic, I didn't see anyone mention the impact that weather will have on mpg. ie: the auto engine runs better in cool rainy weather than it does during hot dry days & of course MPG is affected accordingly. Severe cold also has a negative impact on MPG. ie:
when I lived 8 miles from work in heavy traffic my 1975 Mercury Cougar with 350 cu" V-8 delivered a whopping 8 MPG during Winter. On the highway in the summer running an average 65 MPH it would barely deliver 20 MPG.
Today I drive a 2000 Nissan Maxima with a 235 hp V-6 & in normal city stop - go traffic average 20 to 21 MPG & on the highway in the summer or early fall & running at 70 mph on cruise control get 31 to 33 MPG.
I don't think I could afford to drive the Cougar at today's gas prices which is running $ 4.89 / gal. premium grade thanks to today's value of the Canadian dollar vs the U.S. buck.

Lee


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## Woody Alan (1 Dec 2007)

> I didn't see anyone mention the impact that weather will have on mpg.





> What I can see from my table is that winter has a much more pronounced effect 2-4 mpg less in winter with aircon off,



Confirms my suspicions that I am indeed posting into a blackhole

Alan


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## RogerS (2 Dec 2007)

Lee Brubaker":3pj947r3 said:


> ......
> I don't think I could afford to drive the Cougar at today's gas prices which is running $ 4.89 / gal. premium grade thanks to today's value of the Canadian dollar vs the U.S. buck.
> 
> Lee



Don't suggest you bring it to the Uk then where petrol is closer to 9 $Canadian a gallon (US)


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## newt (2 Dec 2007)

My Audi TT returns 30 mpg, and when driven like a Vicar nearly 40. A.C. seems to make very little difference.


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## Paul Chapman (2 Dec 2007)

newt":3b7a89vv said:


> when driven like a Vicar



Very unlikely - I've heard you're a bit of a speed merchant :lol: :lol: 

Cheers :wink: 

Paul


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## newt (2 Dec 2007)

Paul, what me!


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## exigetastic (2 Dec 2007)

Lee Brubaker":34mxyt68 said:


> After a quick scan of the topic, I didn't see anyone mention the impact that weather will have on mpg.



My car has semi-slick tyres, so during snowy weather I'll have to take my mountain bike, so MPG is much better :lol: :lol: 

The irony of mother nature. When it is cold the air is dense, I get more BHP, but the roads are icy and I can't use them :evil: 

Si


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## White House Workshop (3 Dec 2007)

Lee Brubaker":ljlyb96t said:


> 1975 Mercury Cougar .
> 2000 Nissan Maxima


Hardly a valid comparison! Really old technology versus new. The 1975 Cougar is just a big muscle car whereas the 2000 Maxima is optimised as a sporty family saloon. If you want to compare, I had a 1960's Lotus Cortina that had been 'breathed on' - easily beats the Maxima on fuel consumption (40 mpg on a run and in those days _legally_ driving at 90-100 mph, 32 around town) and close on performance (0-60 in well under 10 seconds), but then it's nowhere near as heavy and isn't fitted with all the latest safety gizmos. Probably doesn't have such thick heavy and comfortable seats, either.

I do agree that cold weather affects automatics, though, as the temperature sensors take longer to kick in. My nice modern BMW won't go out of 3rd gear into 4th until I'm doing 45+ mph in cold weather and won't go into 5th until 70+. However, over 4 years of driving I averaged 29.7 mpg (according to the onboard computer) which for a fast and sporty 3 litre car is pretty good, imo.


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## Taffy Turner (5 Dec 2007)

I am by no means an expert on this, but it strikes me that if a car is being used around town, with a lot of idling, then using the aircon will significantly reduce the mpg, as the engine has to idle faster to power it (try switching the aircon on and off while waiting at the lights if you don't believe me).

Also, say the aircon requires 1 Kw to power it (I have no idea if this is correct, but use the numbers for an example), and at idle, the car engine puts out 3 kw, so the aircon is taking 25% of the total output.

However, at 70 mph, the engine is probably putting out say 80 Kw, so the aircon is only taking 1.25% of the engine's output.

So in summary, around town, or at low speed, it is cheaper to open a window. At motorway speeds, it is cheaper to run the aircon.

Tonight's homework project is to verify if my assumptions regarding power usage are reasonable.

Regards

Gary


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## Vormulac (6 Dec 2007)

I've just bought a 3.0 Legacy and driving mostly around town sensibly I seem to be getting about 15mpg (eep!).

I saw that Mythbusters episode about the window vs aircon test too and as someone who used to work in the autotmotive industry on such things as engine efficiency and airconditioning systems I nearly choked when they announced (with a straight face, you have to admire their acting ability) that you were better off with aircon than an open window. Mythbusters - proudly sponsored by American Petroleum Corp... :lol:


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## cubleymatt (6 Dec 2007)

ok heres my 2p's worth, 

i have 2 cars, the first an 9 year old Audi A6 2.5tdi and the second my good old S3 LWB landy with a 3.9 V8 fitted, 90% of the time we use the Audi, and its rare we use the landy for much, but i know when i do that i dont want to know the MPG, i think double figures would be nothing but a dream !! but then there is the noise of the V8, makes up for sooo much !!  

the Audi how ever will do 40+ on a run and about 35 around the lanes and small towns here, now the aircon, the audi has a little trick up its sleave there, it you sit with the car running and the aircon on and blip the throttle you can hear relay click in the car, this is the car turing off the aircon pump while you accelerate and it will click back in when the motor is not under load. so when you out on the road the pump is out when the motor is needing power and in when there is power to spare. i have tried running with the air con on and off and it only makes 1 or 2 % difference to the mpg, add that to the smoothness of the v6 in the Audi and you can see why i still have it after 8 years ! 

Im sure that my audi is not the only car to do that and i wonder if thats why there is some people that see little difference in the mpg with the AC on or off ?? 

just a thought 

Matt


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## andycktm (6 Dec 2007)

1.Early aircon unit's = (proberbly) very inefficient.  
2.modern aircon unit's = (proberbly) very efficient  
3.Answer = **** (definitely) sticks :lol:


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