angrychair

I have a 2012 JK Rubicon, it has 35" tires and 4.88 gears, from what I was told by seller.
I have verified with the Superchips Flashpaq the tires and gears are set accordingly, however I have checked speed with GPS and have noticed a 4-5mph discrepancy...
70MPH on speedometer is actually about 65mph.
For testing purposes, which way should I adjust the gear ratio to see what corrects it out?

I've done some math but I still can't come up with what gear ratio I actually have or what is going on. I'm just going to have to drive it and check rpm and gps speed.

70mph x 35 tires x 4.10 gears x .84 final drive = 2314.37 rpm
65mph = 2149.06


70mph x 35 tires x 4.56 gears x .84 final drive = 2574.66 rpm
65mph = 2390.17


70mph x 35 tires x 4.88 gears x .84 final drive = 2754.66 rpm
65mph = 2557.9


70mph x 35 tire x 5.13 gears x .84 final drive = 2895.78rpm
65mph = 2688.94

resharp001

You're not going to do anything with the gears program (I presume already programmed to 4.88, but you have a manual anyhow). Simply keep adjusting tire size in the program until the speedo is right. IE, your 35" tires aren't really 35". It doesn't matter what you plug in the programmer in that regard to make the speedo correct. The fact your tires aren't really going to measure out to 35" also makes the math above a bit off. If seller said it was 4.88, then it likely is. You can always rotate the tire and count driveshaft revolution, or even open the diff and count gear teeth, but for just getting the speedo right in a manual tranny, just keep tweaking the tire size in the program.

karls10jk

Correct, it may take setting it at 33.75" to get your speedo right and even then it may be a little low on the low speed side and a touch high on higher speeds- set it for where you're comfortable. Mine is pretty close at 50-ish and around 70 it is 2mph slow, but a little slow is fine with me.

TheDirtman

to get the proper tire measurement, measure front the ground to the center of the hub and multiply x2. This is your actual running tire size.

angrychair

that makes sense. ground to top of tire is 34" but ground to center is closer to 16.5, thanks, newbie and didn't consider that.

bbolander

When I did mine (tires only) I used the new diameter from the manufacture, but then like others, I drove along with an independent gps and adjusted the tire size until the mph closely matched. Took a few times.