2012 Jeep Wrangler: DIY Speedometer Recalibration with AEV ProCal
September 3, 2012
Some months back we took our 2012 Jeep Wrangler to the dealer to have its ECU reprogrammed to correct a speedometer error brought on by the fitment of larger 285/75R17 BFGoodrich Mud-Terrain T/A KM2 tires.
No parts were necessary, but we were charged $85 for one hour of technician labor, the minimum charge. It worked perfectly for a few months, but the error returned just after I installed our Superwinch. Apparently the recalibration was forgotten when I disconnected the battery.
This time I'm going to do it myself with the AEV ProCal device I used to rescale the TPMS trigger point last week.
Believe it or not, a tire has three distinct radiuses, aka radii. The most obvious is the unloaded radius, the radius you'd get when measuring a tire assembly laying on the ground or with the car jacked up in the air. Then there's the static loaded radius (SLR), the number you'd get by measuring up from the ground with the vehicle sitting normally with the usual sidewall bulge at the contact patch.
But the one that matters to the speedometer calibration is the dynamic loaded radius (DLR), the axle-to-ground distance that exists only when the car is being driven at speed. If you don't think DLR can be significantly larger than SLR, then you've never seen top fuel dragster do a burnout at a drag race.
As you can imagine, measuring DLR directly is not possible with simple hand tools, but the AEV ProCal gets to the right answer in a pretty simple way: they have you measure and input a diameter instead.
The diameter they want is pictured above. By measuring from the ground up you get one part static loaded radius and one part unloaded radius. Since DLR falls between these two extremes (on street cars, at least) the end result is a diameter that's close enough to two times DLR for our purposes.
The AEV ProCal needs the diameter to be rounded to the nearest quarter-inch, so this sort of tape measure is more than accurate enough.
Adding 3 inches for the body of the tape, our tires measure 32 inches. When I measure the unloaded diameter I get almost 33 inches, which illustrates the magnitude of error I'd have introduced if I hadn't measured in a way that takes the deformation at the contact patch into account.
A little math is required before I can use this measurement to set the dip switches. My 32-inch result goes into the left hand side of the equation on line two, which returns an answer of 8.00. This boils down to an A dip switch value of "8" and a B dip switch value of ".00"
With the preliminaries out of the way, the tire size dip switch settings shown above are finally complete and ready to transfer into the ECU via the OBD-II port.
Incidentally, there's another page of possible dip switch settings with a different equation that covers tire sizes between 39 and 54 inches in diameter.
The upload procedure is dead simple: turn the key past "acc" to "on" without starting the engine, then plug the ProCal into the OBD-II port and wait about two seconds for the horn to honk twice.
But tire size alone isn't enough to make a speedometer read properly. The computer also needs to know what axle ratio you've got. We have not yet changed anything in this regard, but making sure can't hurt, especially when it takes no more than a minute or two.
So I dial up 3.21:1 on the dip switches and plug it in one final time.
Bingo. Problem solved. The speedometer now reads spot on.
From here on out we'll have no trouble making further calibration changes when we change the axle ratio or tire size later on. We'll no longer have to visit a dealer and shell out cash if the ECU forgets again for any reason.
Best $149 I ever spent.
Dan Edmunds, Director of Vehicle Testing @ 19,754 miles