If I install bigger tires, will my speedometer read wrong?

[khaiwi]

Bear in mind that if you change your tire size, your speedometer will not be accurate. To compensate, you can change your speedometer gears. Randall Groh wrote this handy table:

Mark Tener wrote:

Just got my spiffy new factory service manual for my spiffy almost new 97 XJ. Saw in the chapter on the t-case that there is a removable 'speedometer pinion' gear. Called my local Jeep dealer and asked about it and they said that there is quite a selection of number of teeth to be had. Like from 29 up to 41 with a few odd skips in between. My question is: if I put on larger tires then instead of putting in different ring/pinions front and back to get the proper speed and miles...could I just put in a different speedo gear with a different number of teeth? If so, does anyone have any calculations or knowledge on which number of teeth to use for which size of tire? I know what putting larger tires on will do to usable power but I'm more concerned about getting the right speed and miles. TIA Mark

Mark, I own a '97 TJ and ran into a similar question. It turned out the the dealer that I worked with had the speedometer gears in their computer based on tire size and axle ratio. Below is a chart that I created so that I would know the best gear to use if I change to a non-standard tire.

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 33x12.50R15C 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 Tire Size Tooth Part
Ratio Count Number

3.0 30x9.5R15 28 52067-628
3.0 All P225 R15/R16/R17 29 52067-629
3.0 29x9.5R15, 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 30x9.5R15 33 52067-633
3.5 P225/75R15 34 52067-634
3.5 All P205 R15/R16 35 52067-635

3.7 30x9.5R15 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) * aspect ratio) * 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) * .75) * 2) + 15 = 28.29"

In my case, I wanted to upgrade to a 31x10.5R15 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. Plugging the other numbers into the 'ratio' formula one gets:

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

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

[Bulldozer]

Well Done Dir Gen :behold1: this is a great help for all by answering all the Q in this subject :bravo: