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Tire Diameter/Speedo?


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I'd like to change the size of my rear tires when it's time to replace them. (with a higher aspect tire)


Are our speedos affected by tire diameter?






Only if you change the diameter of the tire. For example the OEM tires on my car are 235/55/17 which is a 28 inch diameter tire. If I change my tires to a 26 inch diameter tire then my speedometer will be off. To find out how much use this tire size calculator.




Enter in the OEM size tire that came on the car. In the second set of fields enter in the tire size you want to run. Rule of thumb is no more than a 3% difference in the diameter. So for my size tire the largest tire I should run is a 285/45/18, both OEM and replacement are 28 inchs in diameter.


Having said all this you can run a different diameter just be prepared for the speedo to be off slightly. Most aftermarket handheld tuner devices such as the SCT Xcal have tire size adjustments to correct this.




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Be careful everyone on those SCT tuner calibrations. It has been shown by other folks that what is, for example, labelled "Stock" on the Bamachips tune is actually off by 5-10 rotations per mile. I recommend that you check the manufacturer of the tire and get the rotations per mile setting. Even those settings could be conservative by as much as 3%. The manufacturers try to error on the side of low MPH so that when you think that you are doing 60, you're actually doing 57. It has been shown by someone on another forum that for example the correct setting for the 18" wheels on the GT should be 760 Rotations per mile, not the 750 stated by BF goodrich. The reason why you might want to calibrate is not only for the accuracy of your spedometer, but also because if you don't adjust it, you will be over reporting your mileage by 3%.


I encourage you to set your watch and time the distance using mile markers. Set your cruise at 60 and see if it takes exactly a minute between markers. If your odometer reports a mile before you get to the marker, you might be adjusted too low in roations per mile.

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