Very surprised that I think only Spectric has mentioned tyre pressures, and servicing. Both have a big impact on fuel economy. As for cost if you have a real gas guzzler then you could look at an LPG conversion. I have a 1994 Jeep 4 litre. Converted to LPG the mpg is unaltered at about 20, but much lower emmissions and it only costs 80p/litre. Conversion is not cheap, probably around 2k if you have it done for you, half that if you do it yourself. Quite straightforward if you know what you are doing, but you would then need someone to certify that it is safe and complies with the various regs. Not a legal requirement bizarrely, but your insurance company may insist on it. Soon get your money back on a vehicle like this from the savings on fuel cost. Availability may be an issue in some places, but you can download an app which tells you where all the stations are, and the latest price.