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When our customers consult with our sales staff regarding lift kits, wheels, tires, and additional needs that come with it, one of the most immediate needs is a speed calibrator. The reason it is almost always necessary is because when you go from a smaller tire, to a larger tire, the larger tire has less revolutions per mile than the smaller tire. For example, if you currently have 33" diameter tires on your Jeep, and you go up to 37" diameter tires, your tire revolutions per mile will go from 611 revs per mile to 545 revs per mile. To take it a step further, this is how that would effect your speedometers accuracy. If your speedometer showed 60 mph with the 33" tire, with the 37" tire, you would actually be going 67.3 miles per hour.
There are other problems created when your speedometer is not properly calibrated. First to be mentioned, which is pretty understandable after the explanation regarding the difference in revolutions per mile, is that your odometer would be inaccurate. Yes, your odometer would show LESS mileage than actually driven, but its not good like some people would think. The reason is because of the other issue created by not having proper calibration. When your speed to rpm ratio does not line up with the factory programmed shift points, your vehicle won't know when to shift correctly, potentially putting more strain on your engine and transmission, making way for more expensive problems down the road.
We offer the Rough Country speedometer calibrator for the unbeatable price of $139.95. It is an external programmer that plugs into your ECM and reprograms your computer inputting in the correct specs for the larger tire. Once reprogrammed, you can unplug the calibrator, and keep it in case it has to be used again.
We hope you found this blog to be informational and helpful. And if you have any further questions, feel free to give us a call at 713-682-1085