Speedometer Gear Applet

by Terry L. Howe

This applet calculates the proper speedometer driven gear based on Jeep model, tire size, and ring and pinion. It is assumed that the Jeep is has the stock speedometer and transfer case to perform this calculation. The program has been very accurate based on my research and experience, but I won't take any responsibility for your speeding tickets :-). Check the caveats at the end of the page.

Simple Speedometer Recalibrations

Jeepers are perpetually modifying their vehicles. Sometimes, these modifications make speedometer recalibration necessary. Any change in the ring and pinion or tire size can throw off the speedometer. The simple way to calculate the proper speedometer driven gear is by setting up a ratio. The ratio should look like this:


newGear * newTire = oldGear * oldTire

or

newGear/newRingAndPinion = oldGear/oldRingAndPinion

Solve this ratio for newGear to determine the proper tooth count for the speedometer gear. For example, if you had 225/75R15 tires (approximately 28.2" in diameter) and you swap in 33s, you would set up the following ratio:


newGear * 33 = 27 * 28.2

newGear = (27*28.2) / 33

newGear = 23.07  (23 tooth gear)

The calculation for a ring and pinion swap from 2.73:1 to 4.27:1 would look like:


newGear/4.27 = 27/2.73

newGear = (27/2.73) * 4.27

newGear = 42.23 (42 tooth gear)

Determining the old speedometer driven gear is very simple. All that needs to be done is remove the speedometer cable (typically with a pair of pliers) and remove the speedometer gear housing which is normally held in by one bolt or his threaded into the transfer case tail housing. Newer Jeeps come with an electronic speedometer sending unit that uses a wire attached to the tail housing instead of a cable.

Complex Speedometer Recalibrations

If you are like me, you may have swapped in a used transfer case from the junk yard, regeared your axles, and put on new tires before you decided to recalibrate your speedometer. A simple ratio doesn't cut it anymore, so you have calculate the right speedometer gear at a lower level. A Jeep engineer might use a formular similar to this one to calculate the speedometer driven gear:


DrivenGear =  (DriveGear*DiffGear) / (TireDiameter*MilesPerInch*Pi*SpeedoRevMile)

The DrivenGear turns the speedometer cable. It is driven by the DriveGear inside the transfer case. The DiffGear is the ring and pinion ratio (e.g. 4.10 for 4.10:1). In this formula, it is assumed the TireDiameter is in inches and this is convert this to miles. The diameter of the tire in miles is muliplied by Pi (3.14 or so) to get the circumference. Every speedometer should have a sticker or writing on the back with the number of revolutions of input it expects per mile. This is typically around 1000, in fact CJs with the Dana 300 and the Dana 20 transfer case have a speedometer that expects 1000 revolutions of input per mile.

Easy Speedometer Recalibrations

If all this seems to complex, get yourself to a speedometer shop. A good speedo shop can recalibrate your speedometer quickly and inexpensively. I know hard core gear heads that don't bother recalibrating their speedometers since it is so cheap and easy to go to the speedometer shop.

Sources

Some of the inforamation to put together this page was obtained from Dan Barbieri and others on the Jeep mailing list. I also refered to Bruce Bowling's speedometer page http://devserve.cebaf.gov/~bowling/speedo1.html

Caveats

The information for '45-'71 CJs is solely based on a transfer case I have from a '49 CJ-3A. It is a dual lever Dana 18 and it looks to me like the parts in the speedometer area are the same for all the early civilian and military Jeeps. The program might not work for later single lever Dana 18 equipt CJs. Also, I have no data on the speedometer gears used with the Quadratrac CJs in the late 70s and early Wranglers equipt with the NP207. I assume they are the same, but it is likely, particularly with the Quadratrac, that the drive gear may have a different tooth count. If you have any information, please send me an email. Thanks.

April 25, 1997