Jeep speedometer Calibration
When changing gears or even tire sizes, the speedometer calibration will change. Jeep Cherokees have a gear driven speedometer sensor even after the speedometer head was changed to electric. A gear dives a pulser unit that sends a signal to the dash.
The gear is mounted in the transfer case on the 4wd models. There are two different gears used. There is a short shaft gear used in the 1993 and later Cherokees and a longer shaft use dint he earlier versions. The long shaft version is the same gear used to drive the cable speedometers used since the 1970’s.
The gears have different tooth counts to provide different ratios to calibrate the speedometer. They are also color coded.
To determine the gear you need you can reference a chart of tire size and gear ratios like the one on Marcus Ohms site. You can also calculate the one you need if you have an accurate speed reference like a GPS or a measured mile.
To select the proper gear for Jennifer’s Jeep, I used a GPS to get a speed reference. I drove at several steady speeds and recorded the indicated speed on the speedometer and the actual speed given by the GPS. Note that most GPS units only update the speed every three seconds, so you have to maintain a steady speed to get an accurate reading.
In this case I found that the speedometer showed 70 when the GPS showed 57 MPH. 55 MPH on the speedometer showed about 45 on the GPS. This gave me a ratio of about 1.2 to 1.
Next I pulled out her drive gear to see what tooth count it was. The gear is held in place by a clamp and a single bolt. There are four different ways to turn the housing so pay attention to which slot is toward the bolt when removing the housing. You can count the teeth but the number is stamped o the gear as well. In her case is as 29.
29 times 1.2 gave 34.8 or rounded up to 35. This matched the chart on Marcus’s site so we set about finding a 35 tooth gear.
Prices vary widely for these gears. My supplier only stocked the short shaft so we had to look elsewhere. Surprisingly the cost from the Jeep dealer was very reasonable and about half the cost of many aftermarket venders. Jennifer was able to source a used on from Ebay for half that however.
Hers came from an AMC Javelin demonstrating the wide variety of cars and trucks that use these same gears. The 35 took gear is coded orange.
To install the new gear, I had to rate the housing to a different position. The gear tooth range for each position is clearly marked o the housing so it is just a mater of making sure it is aligned with the right slot.
Now she has an accurate speedometer reading for her 33” tiers and 4.10 gears.
JEEP NP 242J TRANSFER CASE REAR BEARING RETAINER WITH SPEEDOMETER HOLE # 25988
Auction Ends: Thursday May-23-2013 11:51:07 PDT
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