nthinuf

Umm, huh?

Grab a tape measure, go outside, measure your tire, plug the 'actual' height into the chart.

SilverJK12

I know that. I believe some ppl are looking at the tire size 35x12.5x17 or whatever into the chart. When actually they are 34s. So the chart can look deceiving. If you look at actual size of the tire then most ppl regearing are over gearing in essence. Most people with 35s with 6 speed go 4.88. 5.13 with auto based off a 35 on the chart. If you look at the actual tire height or "chalk test" it's a 34 or less. This puts the 4.88 or 5.13 with that tire height in the red. Get it now? Just saying to get facts straight!

MrDillClinton

How does the new Pentastar engine and new 5 speed auto change things?

I just ordered a 6 speed with 3.73's....I don't really care to go over 32" or 33", but I'd like to know if any of the numbers change significantly. I'm sure alot of other guys purchasing 2012's would find any info helpful, too.

SergeantChuck

I thought along these same lines. I have never re-geared one of my jeeps. I just stuck with the 32s and had a standard and all was well. This time I bought an automatic and decided to run 35s(34.5").

I had seen the chart in the FAQ area as many people on the board reference it. Based off similar comments like you made I decided to take the Jk out on the highway and run her at 70mph and make a mental note of my RPM. I was running about 1920rpms. I went to the chart and looked for where 1920RPM was located and 34" listed 1958rpms and 35 listed 1902rpms. I was about dead in the middle of the two tires sizes.

I then follow the chart over to 5.13s If I run dead center of a 34" & 35" tires I will be running about 2420rpms at 70mph.

I will be changing my gears out soon but until then I run with the overdrive off and the 2500rpm are feels real good. So I think I will go with 5.13s

This is how I determined what I would go with.

JPop

I'm hopeful to get a 2012 chart done soon. I plan to be buying one soon, just some other stuff getting in the way. Most of all just getting a test drive in when there is a 2012 Rubicon on the lot.

The Pentastar is vastly different than the 3.8. You make the same power with a Pentastar at 10-15% lower cruise rpms. The automatic transmission is vastly improved, not leaving much daylight between it and a manual transmission.

So as soon as I get some road time with a new JK I'll be able to get up a new chart. Hopefully by years end.

DunnyBunny

Awesome. I've been looking for a chart like this. Thanks a lot. Just got my jeep so kinda new to all this

JPop

Here's a real loose chart. Posted for reference only, and I'll clean it up over the next week or two.

I did do some editing this morning, so it should be pretty close. I'll wait for some feedback and get it finalized.

69mach1

Actually to get the correct tire height measure from the ground up to the center of the axle and then double it. That will get you the actual running height of the tire, then plug that number into the ratio chart.

Myself I have 1/2 worn 37" Goodyear MTR's, on the Jeep they measure just a hair under 36" ground to the top of the tire, but measure ground to center of the axle and they are 17" x 2 =34"
I'm guessing when I get new 37 MTR's ground to axle center is going to be more like 35 or 35&1/2.

_J__C_

A tire will always measure less when it has weight on it. Why don't you use the circumference to get the tire height? Say you have a true 35, but with weight its 33.5, your still turning over a 35.

69mach1

I believe if the tire was a perfect circle and only a tiny patch contacted the ground then it would travel the 35" circumference, but if you think of the "flat spot" where it is contacting the ground is maybe ~ 6 or 8" and that is going to affect the total rolling circumference Not 100% sure but I think it's something like that...