Well, I just went out and bought a diesel truck and then switched the gear ratios in it to get better MPG. The truck is an older model('86) and it had 4:10 gear ratio and I dropped it down to 3:08's which the truck originally got 18MPG loaded(4ton on trailer) or unloaded it didn't matter, now it is getting closer to 29MPG. Plus I have 33" tires which also increases the overall gear ratio making it higher still.
Most people can modify their own vehicles gear ratios and get better fuel economy for about $400 depending on the vehicle of course and whether or not they have actual differentials to make this easier to do. A buddy of mine put larger tires on his front wheel drive vehicle to increase the ratio and he added a few MPG by doing so, larger tires increase the ratio which then keeps the RPM's down at highway speeds. This probably won't help city drivers cause it takes more power to get the larger tires to move or highway drivers that have a lot of hills but would help for people that drive flat or almost flat highway a lot. Just remember that increasing tire size also means you will need to get the speedometer re-calibrated to have the right speed readout and odometer readings.
I had read about people modding their Toyota Prius's so much they are getting over 100MPG!!! Hyper milers or something like that they are called. They strip these cars down to just the bare essentials, removing seats mirrors, and any other part that may cause wind drag. They drift behind other vehicles so close that it is no way they could ever get stopped in an emergency situation and other techniques are used. Just surprising people will go that far.