More Economical

[Niall]

Well maintained engine and car makes all the difference too.

Can't stress this enough, clean air filter and oil, good bushes and free brakes are worth 10mpg on most diesels. 

If you're interested in modding for economy then it's pretty much as others have suggested: longer gearbox, reduce weight, lowering, etc. If you want to go mad then full undertrays, advancing the timing, fmic, straight-through exhaust and lightweight race wheels are in order.

Basically what you have described there is a race car and all of our modified cars which have been modified like this should actually be praised for saving the planet, being cheap to run and eco friendly.
Ill use that to justify it to the missus next time she kicks off