Am going to buy a 2015 f150 regulsr cab or exrtended with Ecoboost. The 2014 data shows lots of options for rear end ratios. What do you guys and gals suggest for the best gas mileage?

 

What are you going to use the truck for? If your towing or carrying a load of any decent weight, I would say 3.73. If it's just for commuting to work, a 3.31. A medium good all around rear end is a 3.55.
All depends what your going to do.

 

Exactly what Jeffinthebag said. Keep in mind if you plan to modify wheels and tire sizes, or install a lift or change the exhaust you will completely change the mpg and the performance of the truck. In that case I'd go with the 3.55's