I have a 2004 F-150 FX4 on 35x12.50/20 but im looking to upgrade to 38x13.50. I currently have 4.10 gears and im averaging about 11 mpg. What would be a good size gear to go with to possibly help a little with gas milage if i stay with the 35s or what gear would help maintain the curent average with 38s.

 

4:56=35s
4:88=37s. =38s

 

X2