Please hear me out before leaving.stupid replies.

I want to understand the meaning behind regearing. Is it the size of the.tire or the weight that affects it?
I ran heavy 36s on 3.55 with a 5 speed for some time and it drove alright. I plan on going to 38s, and im wondering how that will feel compared to the 36s. If i get some light tires, would it feel the same?
I guess what i want to know is if its the diameter or the rotating mass that affects the gearing. Ill be regearing anyway, but im curious.about the science behind it.

It's the diameter that directly effects the gearing ratio. Say, for example, you can call stock tires(28.5ish") as giving you a 1:1 final ratio. By moving to 36s divide 28.5/36 to get .79, according to that number you have only approximately 79% torque as before. Other indirect reasons for feeling loss of power would be rotational mass(more power to get them rolling) as well as rolling resistance(the wider the tire the more it tends to resist rolling.) As for feel...if you went from 36s to 38s it'd only be a 6% decrease in final ratio. Regardless, most people regear by then, it takes strain off of drivetrain parts "upstream" of the axles.

Use the Gear Calculator at www.Grimmjeeper.com to see the affect of larger tires on your engine RPM's and MPH.

diameter affects the gearing. but most people will gear deeper to help compensate for the weight of the tire as well.

If the factory gear provides a 1:1, how do you calculate the re-gear?

Do you need to re-gear both differentials at the same ratio exactly?

I'm going from stock 98XJ Limited wheels, which I think is 225/75-15 to 31x10.5R15. Assuming some added weight from suspension, heavier tires, bumpers and other stuff, how do you make up for the extra weight in the calculation?


Thanks.

The 1:1 is a figurative number, I'm just using that for "stock" so you can calculate against that. Whatever your new ratio is divide your current gear ratio by it to get an *approximate* "new gear ratio." Example: those stock tires are about 28" so 28/31=.90. Then if your gear ratio is 3.55(most common) 3.55/.9= 3.94 so at that point you have to look at your available gearing and choose what's closest or best for you. 3.78 or 4.10s are options. This is a purely mathematical way of looking at it, it does not account for extra weight. Generally a few hundred pounds extra isn't going to affect much, but you would want to go to a lower (numerically higher) gear to make up for extra weight. Keep in mind though, having a final gear ratio lower than stock will negatively effect your highway driving. Anyways use that gear calculator posted above


Also forgot: you *do* have to regear both diffs at the same time if you plan on using 4x4. If geared differently the front and rear tires will be trying to spin at different speeds and it'll likely blow up your transfer case.

Thanks - super helpful...