Hey, friend has a 99 2500 with a Cummins and also has 35"s on it with stock gearing. (3.55s) I believe. I know on my Wrangler there isa plastic speedo gear which I changed in the transfer case in order to get the speedo to macth the actual speed. Anyone know if this is the same for this truck and also is there any kind of chart out there that tells you what speedo to use depending on tire size/ratio. Or maybe that is all wrong and its electroically controlled and I will need a programmer to change it. Thanks.

 

No need to do that on these trucks.

The speed is read off of the tone ring that is in the diff housing. Changing gears has no effect on it, as it it reads the actual axle rotation speed, not driveshaft speed. The only thing that affects the speedo reading is a tire size change.

This can be corrected with either a - the dealer recalibrating the ECM or b - get a TruSpeed calibrator or a programmer (like a Smarty, Hypertech, Superchips, etc) and be able to adjust it yourself.

I'd go with b, it's a lot more flexible and you can get you speedo dead on.