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Old 09-01-2014, 09:05 AM
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Speedometer calibration

With bigger tires as we all know, the speedo becomes inaccurate. Mine is off almost 10mph all the time. It says I'm doin 10mph slower than I'm actually goin, so my question is How do you calibrate it for the larger tires, or where to take it to have it done?
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Old 09-23-2014, 04:06 PM
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Re: Speedometer calibration

Me too!!! Last fall I changed tire size to 245/70R17, stock is 235/65R17. Had speedometer checked a couple of weeks ago. It is off by 4 MPH at 25 all the way to 6 MPH at 70. I thought the difference would be a uniform percentage off, but it is not. At 25 it is actually going 29 MPH, that's 16% higher than the speedometer reads. At 70 it is going 76 MPH, that is 8.5% higher. Hardly a uniform percentage. Here's another factoid. The speed was off before I installed the new size, I did not have it checked so I don't know by what amount. I did perform a cursory check myself with a stopwatch on a measured mile course and came up with about 5 MPH off at 60 MPH. However, that was not using a radar gun as they did when I had it checked recently. I guess I'm saying the tires might not be the only issue I have with the speedometer. I took it and the calibration sheet to the local stealer (Craft Jeep - Lynchburg, VA) and I got a brush off and a cursory "It's just the tires. Go back to original equipment." line of bovine scatology.
Sorry, but he didn't listen to the whole story, nor would he. I usually avoid that stealer because they have such a crappy attitude and they usually can't find their rear with both hands, but that's the subject for another review.
The question is, can the speedometer be set so it reads closer to the real speed, or can't it? The above referenced stealer wouldn't say. I have been living with remembering how far off it is so I adjust my use of the speedometer accordingly, but once in a while I forget and find myself passing cars on the highway instead of being passed.
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