I have a 2010 Sport RWD 4.0 Auto, i want to lift the truck 3in and run 20x12s. My current gears are 3.55 and pretty sure they wont push efficiantly. My 1rst priority is gas milage. I dont do a lot of long distance, mainly city. So the first question is regearing essential to have a decent ride? I do plan on reagearing but in not sure if that can be put off for now. Second, would 4.10 gears be better or 4.56 gears be better.

 

Use this: https://www.crawlpedia.com/rpm_gear_calculator.htm

20" x 12" ????

The 4.0l SOHC engine makes best torque/power at 2,900rpms, so in Overdrive you don't want RPMs under say 2,400rpm
Lower RPMs does make for better MPG BUT..............if you are lugging the engine MPG goes down, so you need to stay within 500-600rpms of peak power or it won't hold speed at best MPG so you have to push gas pedal down more and there goes the MPG

You need to know peak Torque RPM of any engine to pick best gearing, that's a BIG variable, i.e. 3.0l Vulcan engine makes best torque at 3,700rpm, 4.0l OHV makes best torque at 2,400rpm, 2.9l is at 2,600rpm, so a big Variable

4th gear is 1.0 ratio
5th gear(OD) is .75 in your automatic

So using calculator
4.10 ratio
30" tires
65mph
1.0 4th gear
= 2985 rpms

In OD, .75 its 2239 so a bit low but thats at 65mph, at 70mph is 2411rpms

So its more about YOUR driving habits when choosing gears, plug in some numbers

The 3.55 ratio on 2WD 4.0l SOHC with 28" tires is setup for 70-75mph in OD, 2300-2400rpms

 

20x12 wheels size.Im not sure if wheel size makes a diffrence for gearing thats why im asking, but i plan on runing 32 or 33s for tires.

 

Wheel size doesn't matter, only tire diameter matters

 

Ok thanks thats what I wanted to know!