i have a 02 f150 with a 6" lift and 35s on it currently. I'm running the stock gears of 3.55s and a SCT tuner, I'm looking to move to 37s in the near future and trying to find out what gears would be better for all around performance and gas since I currently get about 12mpg I'm not trying to drop significantly lower than I already am.

 

Which engine and is it 2wd or 4wd? Auto or manual trans?

 

Once you get into that size tire/gear combo, does MPG's really matter?? To me, it seems that folks that go that route aren't really looking for economy??

But, hey, it's a valid question and I'd say just Google "gear ratio calculator" and have at it... As long as you know the overall height of the intended tire and the gear ratio and the speed you are looking to cruise at, it'll give you the RPM's your engine will be turning..

But, the larger the tire, the heavier it is and more rolling resistance, and on and on..

If you are only getting 12 mpg now, don't think you'll be getting any better with the taller and heavier tires, even with a gear change..

You really only do the gear change to regain the power your existing engine puts out to compensate for the taller tires..

Anyway, good luck!

Mitch

 

It's a 4.6L v8 4x4. And gas mileage isn't my biggest concern as I'm just trying to stay close to where I am and not drop into single digits for the sake of my paychecks. I do want to regain the power loss especially on hills. The tuner helps a good bit so far but you can still tell it struggles sometimes

 

Larger, heavier tires will always reduce fuel economy.

Easy way to calculate rpm with different gears is simple.

new rpm at 55mph = current rpm X current tire diameter/ new tire diameter X 4.56/3.55

 

In order to have equal RPM/speed as stock (using 31" tires), with 37's you would need about 4.25's. I would get at LEAST 4.56's.

4.6 and 4wd is a tough combo, not a lot of low end torque.

http://4lo.com/calc/gearratio.php