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Jeep Speedometer Fixes

Posted in How To: Transmission Drivetrain on November 1, 1998 Comment (0)
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This is a speedometer gear. When axle ratios are changed (or for large-tire changes), it's possible to purchase a replacement gear, which will make your speedometer close to normal.







Here's a typical stock Wrangler tire compared to a popular aftermarket tire. You can see the difference between the two, which will also show up on speedometer and odometer readings.



The speedometer error can be calculated with the following formula:
(test course miles x 3,600)
elapsed time of run in seconds



How is this information used? This data shows that when the Wrangler's speedometer says 60 mph, we're actually driving more than 67 mph. That difference is big enough to cost you a speeding ticket. In addition, once you have obtained this number, you can either do the math in your head or make a chart to keep on your Jeep's dashboard. For example, in the sample cited, the difference is close to 10 percent, so 10 percent of the speedometer's reading was added to estimate the Jeep's speed.

To check your Jeep's speedometer and odometer, look for an open stretch of highway with mile markers.

The odometer error difference is a little easier to calculate: It's simply the actual distance divided by the distance recorded on the odometer. In this example, we covered 10 miles while the odometer recorded 8.2 miles; 10 divided by 8.2 equals 1.22 (10/8.2 = 1.22). This shows that for every mile recorded on the odometer, the car has actually traveled 1.22 miles, or that it has a positive 22 percent error in the odometer reading.

Similar to the speedometer correction, the math can either be done as shown above or the Speedometer & Odometer Calibration Worksheet can be used, which takes you through the odometer calculations. Simply drop in the numbers where shown, and perform the calculations. This information is important if you keep track of gas mileage.

Why don't the speedometer and odometer have the same amount of error? There are several reasons. Factory speedometers are rarely accurate, especially on older Jeeps and SUVs, and because of the manufacturing variation, you'll experience differences in the same make and model. Even odometers and speedometers on brand-new vehicles are sometimes out of sync because of manufacturing tolerances.

Up to this point, all we've discussed is tire changes and their effects on your speedometer and odometer. Axle gear changes will also affect your speedometer and odometer, and the same methodology used to check and compensate for tire changes applies to gear changes, too.

However, off-roaders usually install a lower gear (higher numerically), which has the opposite effect on the speedometer and odometer than a change to larger tires would have. For example, we changed gears in a 4x4 equipped with 33x12.50-15 tires from 3.55 to 3.90. In this case, when the speedometer says 70 mph, we are actually driving about 64 mph. In addition, for a large number of vehicles, if you go to a dealer's parts department and tell them what differential gear you are running, they can sell you an OEM speedometer cable gear that will get your speedometer/odometer close to normal. Obviously, a change to a higher gear ratio (lower numerically) (e.g., going from a 4.11 gear to a 3.55 gear) will have the same effect as going to a larger tire.

If you have a nice rig with larger-than-stock tires (or you've changed your diff gear ratios) and are not sure how fast you're really going, you should figure out your real speed-it might just save you a couple of bucks on a speeding ticket some day!

The Calibration Data Sheet is used to record the information that you'll need to calibrate your Jeep's speedometer on your "data run." Remember that the more data collected and the longer the test runs, the more accurate the results.

Odometer Milepost Time Notes Speedometer__mph
0.0 0 00:00:00
9.0 10 00:09:00
Copyright 1998, M.C. Olmstead

1. Collect the indicated data for the shaded areas and complete.
2. Calculate data elements as indicated in formulas.

• Multiple correction factors by actual mileage and odometer readings to obtain vehicles actual speed and mileage.

• If X is greater than W, reverse the entries.
• If not using a stopwatch, make sure to track time, and be sure to correctly calculate time difference.
Copyright 1998, M.C. Olmstead

Here's the Calibration Worksheet with the data from the example in the text. Drop your own data into the shaded areas, and follow the instructions to calculate your Jeep's speedometer and odometer corrections.

Stopwatch Readings: # min:sec Odometer Calculation: # Miles
Ending Stopwatch/Clock Reading: A 0:08:54 Ending Odometer Reading: T 51,304.8
Beginning Stopwatch/
Clock Reading: B 0:00:00 Beginning Odometer Reading: U 51,296.6
Difference (A minus B): C 0:08:54 Difference (T minus U): V 8.2
Total Seconds Calculation: # Seconds Distance Calculation: # Miles
Minutes (only) from C x 60: D 480 Ending Milepost W 111.0
Seconds (only) from C: E 54 Beginning Milepost: X 101.0
Total Seconds (D plus E): F 534 Difference (W minus X): Y 10.0
Test Course Miles: G 10 Odometer Error (Y divided by V): Z 122.0 percent
Factor (G times 3,600): H 36,000 Odometer Correction Factor: 1.22
Actual Speed (H divided by F): I 67.4
Speedometer Reading: J 60
Speedometer Error (I divided by J) K 112.4 percent
Speedometer Correction Factor: 1.12


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