Calibrating a Jeep CJ or Wrangler Speedometer
When installing larger tires or changing axle gear ratios, you’ll find that your speedometer is no longer accurate.
Older Jeep CJs and Wranglers have small plastic gears in the side of the transfer case that can be replaced with new gears that have different gear ratios to adjust the speedometer.
You can use a GPS to determine your true speed as you drive and compare that with what your speedometer says. This difference will allow you to calculate how far off your speedometer is. If your speedometer is slow by 10%, then buy a plastic speedometer gear with a 10% lower number of teeth.
In newer Wranglers, Jeep uses a digital speedometer and sensor system. It has three settings that can be changed by your Jeep dealer if you can find a knowledgeable technician.
The most accurate way to adjust the speedometer with newer Wranglers is to install a recalibration device. For 1992 to 2006 TJ Wranglers, Superlift makes a device they call TruSpeed.
For the 2007 and newer JK Wranglers, American Expedition Vehicles (AEV) sells a ProCal device that corrects a number of issues related to changing tire size or axle gearing.
AEV says: “With the speedometer left uncorrected, there is a major degradation in the performance of the stability control (ESP), transmission shifting, and engine performance.
But, the ProCal module isn’t just for correcting the speedometer. The Tire Pressure Monitoring System can now be recalibrated to better suit aftermarket tires without triggering the dash light.
And, the ProCal Module makes it possible to activate daytime running lamps, "one-touch" turn signals, enable a temporarily increased engine idle speed for winching, clear engine codes, and even has a feature to help align the vehicle perfectly for optimal ESP performance.”