Changing tire sizes/speedo calibration

HI All!

I will be doing a tire changeover for the upcoming warmer weather. As you all know stock tire size is 225 75R 15. The tires I have picked out only come as small as 235's. My question, is that the tire place told me that it will only set the speedo off about .5 mph. I have heard that the variance is greater than that and I will need to have the speedo recalibrated, not only for the speed but also for the odometer too. My XJ is a daily driver that sees about 35K a yr in highway driving so this is very important that my odo is accurate. Otherwise, I will have to switch to another brand of tire. Thanks to all you fellow Jeepers out there--looking forward to your replies!!

Chuck

It's a proportion, not a constant difference. Whatever the difference is, it'll be twice as great at 60 MPH as it is at 30 MPH.

I made up a spreadsheet on tire sizes based in info I pulled from some tire manufacturers' literature and web sites. Many tire companies include a figure for revolutions per mile in their specs, and that's the most accurate way to judge the effect of a tire change on speedometer and odometer.

A representative 225/75R15 (remember that for any size, different tire makes and models will differ) is rated at 730 revolutions per mile. A representative 235/75R15 is rated at 720 revolutions per mile.

The difference is 10/730, which is 1.37 percent.

The fact is that most speedometers are off by more than that, reading faster than you're actually traveling. With 235/75R15s on your Jeep your speedometer will probably be more accurate than it was before, and possibly right on the mark.

Yes, I agree with Eagle. I my 88 XJ shop manual it has a table showing the speedo gear required by tire size and gear ratio. If you have an automatic transmission then you do not need to change the speedo gear.

I got out my GPS and tried it. My odometer is less than 1% off but the speedo is off my 4.5%. I took out the 33 tooth and installed a 31 tooth gear and my speedo was right on the money but my odometer was reading 100 miles even though 105 or 108 miles had been traveled. I went back to the 33 tooth and just remember at 65-70 mph to add 3 mph to the indicated to get true speed.

Don't sweat it, my odometer was more accurate with LT235/75R15 & a 33 tooth gear than it was with the stock P205/75R15 tires with at 35 tooth speedo gear - the right speedo gear for each tire size.

martin said:

Yes, I agree with Eagle. I my 88 XJ shop manual it has a table showing the speedo gear required by tire size and gear ratio. If you have an automatic transmission then you do not need to change the speedo gear.

I got out my GPS and tried it. My odometer is less than 1% off but the speedo is off my 4.5%. I took out the 33 tooth and installed a 31 tooth gear and my speedo was right on the money but my odometer was reading 100 miles even though 105 or 108 miles had been traveled. I went back to the 33 tooth and just remember at 65-70 mph to add 3 mph to the indicated to get true speed.

Don't sweat it, my odometer was more accurate with LT235/75R15 & a 33 tooth gear than it was with the stock P205/75R15 tires with at 35 tooth speedo gear - the right speedo gear for each tire size.

Click to expand...

I recently went from 235's to 31's. Checking my GPS v. my odometer/speedo. My speedo is off about 5mph and my odometer is off by + 8.5%

I don't know how accurate the odometer is, but on both my 95 and my stepson's 93, the speedometer is almost 100% accurate with 235's, according to my GPS. I have about a 4 mph error at highway speeds running 225/70's, or stock 215's.

Try this link. It's not perfect (read it's limitations) but is quite handy nevertheless.
www.csgnetwork.com/speedocalibcalc.html

you can buy new speedo gears from the dealership and intall them easily yourself. im sure if you search you can find a link to DC's tire size and gear ratio chart.

Chux01Cherokee said:

HI All!

I will be doing a tire changeover for the upcoming warmer weather. As you all know stock tire size is 225 75R 15. The tires I have picked out only come as small as 235's. My XJ is a daily driver that sees about 35K a yr in highway driving so this is very important that my odo is accurate.

Click to expand...

Now you'll only drive 34K a year...

I'm running 235/75-15 Pirelli Scorpions, and have noticed no difference in
either the distance to work, or the gas mileage. I did some looking around
beforehand, and I *think* I saw a Wrangler page that showed the P225
and P235 using the same speedo gear.

In short, I'm "within normal limits"...

And it looks SOO much better...

Den

87xj with aw4, 3.55 gears. I switched to 235x75x15 from stock 215x75x15 and my speedometer said 60mph and the GPS said 65mph. Changed the stock green speedo gear for a yellow one and speedo was within 1mph at 65mph per the GPS.