OK for a 75 f250 c6 2wd I have a 3.5 gear rear diff. Was wondering for fuel efficiency what is the best gear ratio...

 

There are so many variables that lead to "efficiency" it is hard to say what is best for you. Tire size, how you drive, city driving, highway driving, and on and on. There is really no 1 size fits all. And with a 75 f-250 you cant expect alot. If you cange to a higher ratio(lower numerically) you sacrifice alot of what your truck can do. YOU have to decide what you want your truck to do

 

"Best" is a subjective term.

3.50 rear gears (with a 1:1 top gear) is typically a good compromise btwn efficiency (mpg) and performance (launching, towing, hauling).

But throw in the mix of tire size, engine RPM range, topography, and driving habits and it may not suit your particular situation.