Tire Size/Speedometer Tooth ?

[BIGSLVRXJ]

Hey guys, I know this probably sounds like a dumb question but what speedometer gear would I need. I've got P235/75 R15 tires. Also can someone break up what each of the numbers in an equation like that means. (I feel retarded not knowing, but I dont). Like I know the "R" means radius. Oh my stock ones were P205/75 R15
Thanks,
Collin

 

[Yucca-Man]

Hey guys, I know this probably sounds like a dumb question but what speedometer gear would I need. I've got P235/75 R15 tires. Also can someone break up what each of the numbers in an equation like that means. (I feel retarded not knowing, but I dont). Like I know the "R" means radius. Oh my stock ones were P205/75 R15
Thanks,
Collin

Don't worry about changing the speedo gear with such a small change.

235/75R15:
235 - the section width, or width (in millimeters) of the tire. 235mm = approx 9.25"

75 - the profile, as a percentage of the secion width. The sidewall is 75% of the width of the tire; "low-profile" tires that have a short sidewall are 40, 50, 55, etc.

R - Radial

15 - rim size. 15" rim...

Hope that helps, www.4lo.com has a number of useful calculators that can help do the conversions.

 

[BIGSLVRXJ]

So using 4lo's calculator, is the 235 the diameter?

 

[Yucca-Man]

Go to the "Metric Tire Conversion" calculator. As I said eralier, the '235' is the width, not the diameter. Enter 3 of the 4 values and it will calculate everything.

 

[BIGSLVRXJ]

It's looking like at 55 on my speedo its only really goin like 58 so I wont worry about changing it. Thanks Yucca-Man.

 

[Cherokee-Dan]

for future refrence its the gear in the transfer case you replace ive got the website for the chart.

here it is http://www.off-road.com/jeep/cherokee/98/07_jul/speedo/speedo.html

 

[BIGSLVRXJ]

Although I doubt I will change it I may do some highway testing to see how much difference in speed there is. The chart may come in handy, thanks a lot.

 

[martin]

I have a 88 XJ 4.0 automatic and the stock tire size was P205/75R15 and I went to BFG AT LT235/75R15 tires. My stock speedo gear was a 35 tooth. From the chart in my 88 XJ factory service manual says with the 3.54 gears used with the automatic to use a 33 tooth gear for P225/75R15 and P235/75R15 size tires.

I'd change it just so you don't get speeding tickets. Going to larger tires you will be traveling faster. Also I have found out using my GPS that my odometer is right on the money but my speedo is registers similar to yours. I like having an accurate speedometer. I went to a 31 tooth gear and the speedo was right on but i would log 108 miles for 100 actual miles.

It a 10 minute job to change the speedo gear. Just spend the $10 and finish the conversion to the larger tires. Double check the door sticker on your XJ and see what size tires it came with, you may of lucked out and went back to the stock tires.