hiteck

The changing gears is to adjust for changing tire size., right? I'm assuming (i know, i know) that the change is to return the tire size/gear ratio back to or better than stock?

Is there a magic formula or lenghty algorigthm for picking the right gears for a certain setup or particular use (i.e. rock crawler, daily driver, etc...)?

My X (auto) has 4.10 gears and came stock with 225/75/16's. I'm now running 265/75/16's. This has thrown my speedometer off roughly 10%.

More importantly it changes the amount of rpm's needed to reach/maintain a certain speed. If I go up to 33's or 35's these things will be effected again.

I guess my question is how should someone re-gear properly to accomodate tire height changes?

hiteck

In the Q/A sticky it says
So is this 4.88's for rigs with 3.21's, 5.13's for rigs that came with 3.73's, and something else for 4.10's?

If I'm going to run 33's for a while, how do I determine the best gear for my Jeep?

RockCrawln

There are several charts available without having to do the actual math. Here is one:

http://www.4lo.com/calc/geartable.htm

Remember that most are at 1:1, so adjust for overdrive accordingly.

TEEJ

The simple rule of thumb is to go by the difference in tire size...like if your speedo is off by 10%, you need a 10% deeper gear to get back to stock gearing, etc.

Obviously, you pick what ever is available that's closest, typically going to the deeper of two close ratios if you need to pick.

If you cruise at ~ 2,000 rpm at 60 mph stock, add 10% larger tires, so you drop 10% rpm to go 60 mph...thats 200 rpm - or 1,800 rpm at 60 now instead of 2,000 rpm at 60 mph.

If that's worth a $ grand or so to you, get new gears.



If you are going from 3.73's - adding 10% = ~ 4.10's, etc.



If you want more oomph - go deeper, etc....and the new RPM will increase by that change as well....you go to 4.88's instead of 3.73's, you're going ~ 24% deeper instead of 10%...

...so instead of 1,800 rpm at 60 mph on the 10% larger tires, you'd be running closer to 2,225 rpm at 60 mph.

4.10's would have brought you up to about 1,980 rpm from the 1,800 rpm....pretty close to stock feel again.

Considering the costs and down time involved...I'd do AT Least 4.88's instead of 4.10's.

Hope that helps!

2K1TJ

(new tire size/factory tire size) = (new gear ratio/factory gear ratio)

example:
changing a Rubi from stock 4.10's and 32's to 37. What ratio to use for factory-type gearing?

37/32 = x/4.10

4.74 = x

The closest-available new gear ratio for factory-type gearing would be 4.88's. Most people go a notch higher to offset the increased rolling weight of the bigger/heavier wheels and tires an dbetter offroad performance, so that would be 5.13's.

hiteck

So adding a bigger tire and not changing gears will drop the rpm's for any given speed because of the additional tire circumference, and depending on the amount of power produced at the new rpm will effect how it drives compared to stock, right?

Thanks for the replies and links. With so many different gear options and tire options I just wanted to make sure I know what I'm doing or at least have an idea of what I'm doing when I go to a larger tire.

barbourjay

i found 5.13's for 229 for the front and the rears for the same. what else do i need? any better prices out there?

i have an x so i assume i need them for a d30 front and a d44 rear.

wayland1985

This argument is assuming they're all running the same size tire, which they're not. Typically, the 3.21's are installed on jeeps with the 30 inch tires. The 3.73's are on the 32 inch tires, with the 4.10s being an option for towing (in 2007 models)

Gearing is just done to put your engine into desirable rev-ranges to make up for the increased circumference of the tires. Also, it seems people are gearing numerically higher to make up for the added weight of the tires, as well as getting a little more "Pep" from the engine. I don't know how this will effect launching from 1st though. Or how it will affect reverse (I have the 3.21's with 30's. There is no such thing as releasing the clutch fully in reverse. It's too fast!)






I beleive you need an installation kit, and a reputable mechanic. Regearing doesn't seem to be for the beginner (assuming you've never done it before). It seems very messy and very precise.

barbourjay

word, i have a few shops nearby. need to figure out which is the best but i need to get this done soon.

hiteck

So X's with auto come with 32's instead of 225's