Just how much does tire size affect the odometer - Jeep Wrangler Forum
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Old 12-24-2010, 12:24 PM   #1
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Just how much does tire size affect the odometer

My 2004 Sahara came stock with 30" tires and at some point a PO changed them over to 31x10.5x15's. On the surface it doesn't seem like that much of a difference circumference-wise, but I really think that my odometer is off and probably my speedometer as well. I bought my Sahara literally 32 days ago and although I'm loving it to death and drive it as a DD plus "whenever I can", the odometer is saying that I've driven just under 2K miles. This is just not possible - hence my question. Would a 1" tire upgrade really skew the odometer that much? Unfortunately I do not own a GPS to confirm the odometer's accuracy. Should I just bite the bullet and switch the speedo gear? BTW, it's a 5-speed. On the plus side, if the odometer is skewed incorrectly and depending on how long the 31's have been on the jeep, I may have substantially less miles than what's showing on the odometer. Merry Christmas!

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Old 12-24-2010, 12:30 PM   #2
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A 10% tire diameter change affects the speedometer 10%. It takes 5 minutes to recalibrate the TJ's speedometer per the instructions at Changing Your Jeep Speedometer Gear

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Old 12-24-2010, 12:31 PM   #3
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I have the same tire on my "06 w/auto, I checked mine versus the mile markers and found that the odometer was consistently 1/10th of a mile less.
Which would add up to more miles than what shows on the odometer.
There is probably a accurate way of doing it.
Hope this helps.
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Old 12-24-2010, 12:36 PM   #4
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Run with a gps and find out... Most tire sites have a calculator that will tell you how off it is roughly, too....
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Old 12-24-2010, 12:40 PM   #5
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Thanks for the quick reply and for the link to change out the speedometer gear. I think I will try to borrow a buddy's GPS after the holidays are over to accurately see just how much it's off. I'll let you know what I find out. I'm going to go ahead and order the speedo gear - it's certainly cheap enough. Thanks again!
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Old 12-24-2010, 12:44 PM   #6
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Mine is about 5mph slow.
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Old 12-24-2010, 12:47 PM   #7
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This is a link to an RPM Calculator
Engine RPM Calculator

Just enter rear gear ratio, tire height, trans gear ratio and mph and it'll spit out rpms...

Ratios for the AX15 are: 3.83, 2.33, 1.44, 1.00, 0.79, for first through fifth, respectively.
...
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Old 12-24-2010, 06:09 PM   #8
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Here is a basic calculator that tells you how much it is off.

Checking Your Speedometer After Changing Tire Sizes
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Old 12-25-2010, 08:53 PM   #9
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Quote:
Originally Posted by coolbreeze View Post
Mine is about 5mph slow.

Only at a specific speed...your tires throw it off by a percentage. So at 100 mph you may be 5 mph off, but at 60 mph you may only be 3 mph off.
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Old 12-25-2010, 09:30 PM   #10
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Quote:
Originally Posted by Martinez_Coins View Post
Here is a basic calculator that tells you how much it is off.

Checking Your Speedometer After Changing Tire Sizes
That is a handy link! Thank you for posting it up!
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Old 12-25-2010, 09:40 PM   #11
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Mine with my 285's is off about 13% per the charts , mph wise it's off about 8mph at 60.

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