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.