I have kind of a strange 2 partquestion. I have a hypertech programmer and it programs for tire size changes but not rear differential gear changes. Since I have changed both is there some way of tricking this change? What I mean is some way of calculating a different tire size to be input that would arrive at the same combination of tire size/gear ratio. The other part of this is if I just have the speedo done at a shop willthe hypertech undo this?

 

welcome. you need to fill out your signature in your profile so everyone knows what year your truck is, as that will matter in many of your questions including this one. On the later models, the speedometer signal is transmitted from the rear wheel speed sensor (rwss) located on the top of the differential housing. here's what i think, someone will correct me if i'm wrong... The sensor somehow reads the tone ring on the ring gear carrier, maybe a magnet or notch or something. Since its reading the turns of the ring over time, its the same as reading your wheel/tire rotations, and so the speedometer needs tire size to calculate speed and distance. pinion/gear ratio is unimportant. If you have a wire to the top center of your differential housing, you have this. i think the older models still had a plastic speed gear in the transmission, with a cable up to a traditional speedo, and you just have to guess up/down on the number of teeth on this gear to get it as close as you can. i don't know what year this ended.

you want to ALWAYS remove the hypertech program before taking the truck to a dodge dealer, or else you run the risk that they might reflash your pcm and over-write the hypertech. if this happens and you are the original owner of the hypertech and you have a receipt, hypertech will probably provide you with a replacement. if you just bought it used and don't have a receipt, then your out of luck. I don't think any place other than a dodge dealer could reflash over your hypertech.

 

Sorry, it is a 1999 1500 5.9L 2WD. I was led to believe by the dealer that the speedo correction needed to be made through the computer. Of course it would not be the first time they have steered me wrong.

 

the 99 is rwss (computer controlled). if your hypertech will give you the prompt for tire size, you can calibrate it there. that entry will be stored somewhere in the trucks computers and likely never erased. likewise, if you have the dealer do it, it will be stored in the same manner. installing the hypertech program will ask a series of questions for octane, removing some limits, and changing tire size. it will not change anything that you don't tell it to change. i'm not sure what happens to the tire size correction if you remove the hypertech program.

i recently got a hypertech, but mine would not allow me change the tire size. bummer.

 

If you changed the tire size to fix your speedo and then remove the program (put it back to stock) the speedo will go back to being off.

 

If I am understanding you correctly I do not need to concern myself with rear gear ratiochange as far as the speedometer. I do need to program for the tire size which yes the hypertech does do.

 

Yep, as far as I know you don't need to worry about the gearing because that won't affect the readings sent to the speedometer.