Bought '08 JKU from dealer lot and happinstance spoke briefly with previous owner. Said when he got it it had a 4in lift (assuming Rough Country bc steering stabilizer still on it is RC) but he had it taken down to a 2in poly lift with Skyjacker hydro 7000 shocks...he had it all done at the dealer and I have the dealer work orders and parts list print outs but nowhere says anything about recalibrating anything...she sits on 285/70R17's now and would assume she had to have bigger to accommodate bigger wheels ..every rattle and such I've attributed to "it's a Jeep"...oh and it's an auto 3.8...Question, if it was tuned to bigger setup and not redone to for downsizing, what are the signs???...Procal anyway???

 

Check the speedometer and odometer for 60 seconds over a measured mile at 60 mph. Or, check the speedometer against a gps.

Check the axle gear ratio by doing this:

Jack one front tire off the ground.
Put a chalk mark on the tire and the drive shaft.
Rotate the tire by hand 2 complete revolutions while counting the drive shaft turns. The number will be your gear ratio.
A little less than 3 and 1/4 turns will be 3.21 gears.
About 3 and 3/4 turns will be 3.73 gears.
A little over 4 turns will be 4.10 gears.
etc.

 

I do know she has 3.73 gearing...will check the speedo against gps tomorrow, how much difference will there be if she is off???

 

2 Complete rotations? That doesn't sound right. Shouldn't it be 1 rotation?

 

1 tire off the ground, spin tire 2 rotations.
Both tires off the ground, spin tire 1 rotation.


4" lift and bigger tires, right? And you are sure that they did not regear?

 

Because of the action in the differential, 1 tire off the ground requires 2 revolutions.

Turning 1 tire 2 revolutions is a lot easier than making sure 2 tires turn 1 revolution exactly together.

 

Previous owner said it has 3.73. I would assume if the owner before him had 4in lift, and Jeep needed new fender flares (on parts list from dealer) that he ran way bigger shoes at one time.
I'll do the tire revolution you guys mentioned after work today just to make sure.

 

Checked speedometer against gps and they match up, unable to specifically check gearing tonight. So maybe the odd shifting points and "sweet spots" relating to rpm/speed are just unique to this Jeep.
Thanks for all your responses. The knowledge here is incredible and keeps me reading this forum multiple times a day...
Thanks again

 

Hand held GPS or in dash GPS (factory installed)?