So I just went from stock rubi gears to 5.38's. Did the job myself. Everything seems good. Have 100 miles since job. Do I need to reprogram gear ratio. It seems like the speedo is correct. I did not change tires yet.
If the speedo is correct does it need reprogramed to shift differently or should that be correct also? Its a 2008 auto

 

Yes you will need to reprogram the speedo. A Superchips programmer or procal will allow you to enter your tire size and gear ratio to calibrate the speedo which will help with the shift points as well

 

Well I haven't changed tire size yet. So would just the gears mess it up

 

Yes. Tire size or gear changes will effect speedo and the shift points in an auto. Just reprogram it.

 

The reason I ask is because I have a diablo tuner that I like and it has tire size for when I change them but they claim gear size does not affect speedo.

 

Just trying to make sure

 

Just regeared my Sahara to 5.13, you definitely need a super chip or ...... to set your transmission gear changes as well as your speedometer, also remember your Jeep will clock twice (sort off) as many miles if you don't adjust the ODO........

 

I'm rethinking this. I do not know for sure where the speedo gets its info. If it's from the wheel speed sensors for the abs, then gearing would not affect it. If it's from the ouput of the trans or tranfer case then it would.

Maybe someone that knows where the speedo gets its info, will chime in.

 

Not exactly sure where the signal comes from, but my speedo is off about 4mph. I returned everything to stock with my Flashpaq and now it reads wrong. We have 5.38s and 315/70-17s.
I believe programming tire size corrects the speedometer, not gearing, which would indicate the signal comes from the sensors at the wheel.

 

Changing the gears will not affect you speedo. It reads from the abs tone rings on the axle so it measures by tire rotation. I would still recommend superchips for recalibrating both the gears and the tire size.