The engine-building tutorials have all kinds of tips about how to improve the economy of the engine, but even that alone doesn’t guarantee good fuel economy test figures. Note that I said test figures, obviously real world economy depends on how you drive the car and the game has a very specific, uniform manner in which it calculates economy, so basically you will need to set the gearing such that it’s at its most efficient at the cruising speeds, which, given that car and the body, will likely necessitate an overdrive gear or two.
If you want specific information on how to maximise economy, maybe you can post more information. We like to see the parts you used, the gearing, the ventilation, and the engine power graph. In 2005, if MPFI is way better than SPFI, and DI is way better again but horrendous for the budget.