So im told my best option is to regear to boost performance. i belive my stock ratio is 3.55 and im looking at a 4.10 or 4.56. Im sitting on 305/70/17 (summit mud dawgs) tires. it appears im only 2.5 inches taller than my stock tires. Will i get any better highway gas mileage by changing my gears? Truck is an 05 STX 4wd 4.6l V-8. My current average mpg is around 14, and i drive at least half hwy on a day to day basis. Mods are as follows: K&N filter, flowmaster exhaust, throttle body spacer, and a 3" leveling kit. Can this truck get 20 mpg, or am i asking to much?

 

20 mpg is too much to ask. Try downsizing the tires to a 32" and you might have a chance.


4.10 gears aren't that big a jump. You will notice a difference. But it won't be enough to offset your 35" tires.

4.56 gears are pretty much perfectly matched with 35" tires. it will return your drivetrain gearing ratio's to almost stock levels. alot of guys report their city MPG's going back to stock MPG's.



What are your current Highway RPM's at say 65 mph?

 

Actually those 305s are only 33.8". And moving from 3.55s to 4.10s is a pretty nice jump! Nearly 15% steeper when the tires are only 7% larger.

You'd be fine with 4.10s. That will still put you above your stock effective ratio and would be like running 3.73 (actually a tad higher) with stock tires. Plus, you can get factory 4.10s. Factory gears are typically quieter than aftermarket gears.

You could remove that silly throttle body spacer and sell it to help offset the cost of your gears, too!

And about your mileage. Unless you've re-calibrated your speedo for the larger tires, you aren't really getting 14.

 

I installed 4.10 gears in my f-150 2 wd last night. With my 31" tall tires you will be turning 2000 rpm @ 65 mph and about 2450 @ 80 mph. I thought it would make a bigger difference in rpm but its only a .55 diff in ratio. With your tires 4.56 gears sound about right. Bob.: