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how changing tire size effects 06 LJ speedo

3K views 11 replies 7 participants last post by  JeepinHusky 
#1 ·
I have a 06 LJ Rubicon with 37" tires, 4.5" lift, 5.13 gearing and a speedo that reads 68 when actually traveling at 65 mph.

Questions:

1. If I downsize to 35" tires will that increase the discrepancy between speedo & actual mph?

2. Being that my LJ doesn't have an external mechanical speedo that can be easily changed out (like earlier TJ's), what can I do to reduce the speedo discrepancy?

Thanks in advance.
 
#9 ·
Diagnostic Software: Can it Change Speedo Calibration ?



Just Curious if anyone has looked at these diagnostic software packages that come with a blue tooth Diagnostic Port Dongle. Wonder if any of them give access to the settings in the PCM for axle ratio and and tire size (usually spec'd out at revs/mile).


Anyone ?
 
#11 ·
Don't even think Chrysler can change it but maybe 05-06 but doubt it for two reasons

1st rubicon does not have different PCM and non rubicon calibrate with speedo gear not PCM programming

Original Rubi plan was 33 tires pulled at last minute but all 03-04 rubicon were about 8-10% high on mph and odometer reading and Chrysler did nothing to fix it (maybe they liked shortening actual warranty by odometer over reading or perhaps they had no way to fix it with programming)
 
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