3:55 gears to 323 gears

AJ/FormS said
Hang on 273.
That puts the 3.55s pushing more wind , which is gonna take more throttle to do; so IMO not a fair fuel-mileage test.
There is a long standing ratio that says a 10% reduction in rpm, can result in a 5% reduction in fuel mileage. And vice-versa.

Yes it's not perfect but gives an idea, doubt the difference be all that much. If your 5% is right say he get 15 mpg now that would be 15.75 mpg after even 10% would 16.5 mpg. You know how much highway driving for that to equal any real $$$ in the pocket.