2012 Jeep Wrangler Sport: Speedometer Recalibration
January 10, 2012
The speedometer in our 2012 Jeep Wrangler is accurate once more. Even with the big tires, 35 mph is now 35 mph, 70 mph is now, well, 71 mph according to my GPS speedo app ( a free one called Car Dashboard this time) but that's close enough.
My local dealer, Glenn E. Thomas Jeep, did the trick in about an hour -- 50 minutes of which was spent waiting for a service technician to become available. All he needed from me was the circumference of the new tires, expressed in revolutions per mile. A quick check of the BFGoodrich website showed that number to be 630 revs/mile for our LT285/70R17 Mud Terrain KM2s.
For reference, our Wrangler's smaller stock tires rotated 705 times per mile. Yep, that's 12 percent more.
We made the tire swap 1,268 miles ago, so the 12 percent error during that time amounts to 152 miles and won't grow any larger.
I wasn't allowed to go back into the service area to watch or photograph the procedure, but I'm told it was performed through the OBD-II port. Jeep of course offers a variety of tire size and differential options on the Wrangler, so there's a changeable tire circumference parameter built into the ECU programming.
According to what I was told the spectrum is stepped, not continuous (it isn't possible to notch the number up or down in 1 rev/mile increments) and there are upper and lower limits. Our tires apparently sit right at the cutoff because my service writer told me the technician used the last available choice.
For these reasons the service ticket reads "recalibrate speedo as best as possible." This may also be why 71 mph on the GPS reads 70 mph on the speedometer. Close enough, but not spot on.
The service writer also told me there isn't a similar ECU parameter for the gear ratio in the diffs. When we swap out our 3.21 units for something more sensible we'll need to pretend we put on smaller tires instead and figure out what the equivalent recalibration factor would be in revs/mile. Luckily, that will push us back in the direction we just came from.
Total cost: $52.50, all of it labor. Plus tax, of course.
Dan Edmunds, Director of Vehicle Testing @ 7,376 miles