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:wavey: Hey all, just picked myself up a 1998 Ram 1500 and it came with 285/75/16's on it. As I was driving it home I noticed the Speedo and GPS were 6mph off, as in the truck was showing 6mph slower than reality. From my research, 1998 is a tough year because late models needed them to be programed. Mine was built in November of 97, but I can't seem to the speedo gear (if it has one). Does anyone have an idea? Any help would be much appreciated, I hate to see this truck rack up more miles than what it is actually traveling.
 

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I'm pretty sure if its mechanical the speedometer gear housing should be on the driver side of the transmission. If you figure out its digital you will need access to a computer that can change the pinion factor. It takes less than a minute and dealerships rip you off.

Another option is to purchase a hypertech programmer/tire calibrator to dial in whatever tire size you have or get later.

The pinion factor formula would be 63360 divided by (Pie x Diameter of tire) = Rotations of tire per mile. A 285 75 16 tire = 32.83." 63360 / (3.141159 x 32.83) = 614.3 rpm. You have to measure YOUR tire height because every tire company for the same sidewall label is a different height. Make sure the tire is off the ground to stay round when you measure it.

If its mechanical you will have to change a gear. Transmissioncenter is helpful for choosing the right gears.
 
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