tunin will net you AT MOST 5-6 regardless of cars, and thats only because most cars that try and get tunes for MPG are modded and lost MPG due to various tuning issues in the first place if that makes sense...
usually getting your trims down to +/- 2% ltft helps for 1-2 mpgs and beyond that its just about how perfect your cruising tune is. Lean cruise tables can be adjusted but really doesn't help too much.
just getting your car tuned to perfection will net you a couple mpg.
when i had my 3800 fbody, i used to get 29-31 mpg stock (3.42 open rear), then the mods started. when i was done i was getting roughly 11-15 highway (12 psi, 63lb injectors, 411s, 4200 stall converter, no lockup). three weeks of logging and tweaking on every ride i took whether it be a short cruise to class or a long cruise at nights got me up to the 15-16 mpg highway range, i'm not even going to mention city.
40-50 is a LOT. maybe if you ran on 1 cylinder, downhill, with a tail wind, with brake pads removed, on bald 13" tires...