DS/RPM–TIRE SIZE–REVS/MILE–AXLE

         2,167--        30X9.50R15--    706--        3.07
         2,275--        P225/75R15--      741--             3.07
         2,315--        P215/75R15--      754--             3.07
         2,385--        P205/75R15--      777--             3.07
         2,506--        30X9.50R15--      706--             3.55
         2,631--        P225/75R15--      741--             3.55
         2,677--        P215/75R15--      754--             3.55
         2,758--        P205/75R15--      777--             3.55
         2,902--        30X9.50R15--      706--             4.11
         3,046--        P225/75R15--      741--             4.11
         3,099--        P215/75R15--      754--             4.11
         3,193--        P205/75R15--      777--             4.11

The key columns are the driveshaft speed (assuming a 1:1 ratio), and the tire

revolutions per mile. Tire manufacturers generally publish revolutions per mile

for the tires. For example: a BFG All-Terrain T/A 33×12.50R15C is rated at 634

revolutions per mile. With a 3.55 axle ration this yields 2,251 driveshaft

revolutions per mile. If I were to install this tire on my TJ I would tell the

dealer that I need the speedometer gear for the P225/75R15 tire and 3.07 gear

ratio. This is 2,275 driveshaft revolutions per mile for an accurate reading. My

speedometer would only be off by 1%.

If you check with your dealer you should be able to find out what tire and axle ratio options are available for the '97 Cherokee and build a similar chart.

Rob Chaput (rchaput at frontier.net) mailed me the following, which seems easier to deal with:

When changing wheel size and or axle ratios, the speedometer must be re-calibrated in

order to display the correct MPH and odometer readings. On the transfer case there is

a speedometer cable which is driven by a gear. By changing to a gear with a different

'tooth' count an adjustment is made which causes the speedometer to function properly.

In order to determine the 'new tooth count' the following information will be needed:

  Old tooth count
  Old tire diameter
  Old axle ratio
  New tire diameter
  New axle ratio

The basic relationship is:

New tooth count / Old tooth count = (Old tire diameter / new tire diameter) *

                                                        (Old axle ratio / new axle ratio)

The 'old tooth count' can be obtained from a Jeep dealer's parts catalog (Catalog

“97-98 TJ” GRP 21P.) The following table will provide some common

configurations:

Axle Ratio– Tire Size– Tooth Count– Part Number

3.0– 30×9.5R15–……….. 28– 52067-628–

3.0– All P225 R15/R16/R17– 29– 52067-629

3.0– 29×9.5R15, P225/75R15–. 29– 52067-629

3.0– All P215 R15/R16/R17–.. 30– 52067-630

3.0– All P205 R15/R16–…… 31– 52067-631

3.5– 30×9.5R15–…………. 33– 52067-633

3.5– P225/75R15–………… 34– 52067-634

3.5– All P205 R15/R16–…… 35– 52067-635

3.7– 30×9.5R15–…………. 34– 52067-634

3.7– P225/75R15, P215/75R15– 36– 52067-636

3.7– P205/75R15– 37– 52067-637

4.0/

4.1– All P225 R15/R16/R17– 39– 52067-639

”–.. All P215 R15/R16/R17– 40– 52067-640

”–.. All P205 R16/R16–…. 41– 52067-640

Tire diameter for metric tires is determined by the formula:

(((Size in mm./25.4) X aspect ratio) X 2) + rim diameter. For the P225/75R15 the

calculation is as follows:

Size in mm. = 225, aspect ratio = 75, rim diameter = 15

D = (((225 / 25.4) X .75) X 2) + 15 = 28.29”

In my case, I wanted to upgrade to a 31×10.5R15 tire. Using the table, I determined

that the 'old tooth count' for P225/75R15 tire and a 3.73 axle ratio (Dana 44-3) was

36. Pluging the other numbers into the 'ratio' formula one gets:

new teeth count = 36 X (28.29 / 31) = 32.85 = 33 (rounded)

I therefore need to order P/N 52067-633 from the parts department.

 
correct_your_speedometer.txt · Last modified: 2010/06/16 13:42 (external edit)
 
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