Speedometer Calibration

[intemperant]

I finally received my back ordered u7180 for my '01 Duramax. The firmware is at v9r10. The truck has been lifted from stock and has a 3.73 rear end and the new tires are 315/75R16. Mathematically, the numbers verify with what the speedometer says stock compared to GPS (and cops radar).

Imagine my surprise when I put this information into the tuner and my speedo now says I am doing about 7mph when I am clearly (GPS and other cars) doing 21-24mph and it just gets worse as the speedo gets closer to 10mph.

To make a long story short, I am running at the absolute tire size of 32" (rather than 34.6) and I dare not touch the diff settings for every time I do, the speedo registers beyond fast or beyond slow.

I have also found as I run through these settings that the ABS light stays on, and sometimes, so does the parking brake indicator.

The last bit of frustration came when cruise control stopped working. After 4 hours of tuning and re-tuning, I am done and ready to go for a refund unless I make some real headway.

Why can't I put in the size of the tires and differential and expect it to work properly? Why is there no consistency between changing a tire size up or down? Why when I set a considerably smaller tire size to make the speedo closer to correct do all the warning lights come one? What happened to my cruise control?

Please don't tell me to invest more money in a data cable and upload you the default tune. Surely there is an answer or surely you all have found the proper combination of tire size - perhaps a chart of some kind? A decade of doing this and hundreds of forum posts must have some real answer by now, right?

[intemperant]

SO I have given up on this thing. Today I set the tire size to 32" and the speedometer registered slow. So I then set the tire size to 31.5" - tire sizes are in 1/2" increments only on this screen. The speedo registered too fast. So I think OK, all I have to do is figure out a proper tire size that is about halfway between those sizes and I am home free - WRONG!

I set the tire size to 285/75R16 which has an actual diameter of 31.7" and my speedo goes nuts. When I am doing about 28MPH according to GPS, my speedo is pegged at the 100MPH point on my speedo.

Plus, my cruise control stopped working. I am going to return this nightmare. I see no reason to trust this thing with putting a power tune on my truck when by no means can it properly adjust my tire size with any consistency.

I have restored the original backup, my cruise control works again, and I am going to get my $300 back - minus whatever ridiculous restocking fee the store wants to charge. I am done wasting time and money.

[mikel]

Never seen this befroe, any chance the truck was tuned inthe past?

[Trotorx2]

I've had problems getting the speedo calibrated after installing a tune. When I changed from 245's to 265's it was off and took forever trying different settings, never got it correct. Now with my 265 17's it is off and to set it by the settings on my 7186 it sets off the ABS light. I just have to remember I'm going faster than the speedo says.

[rydnhi4x4]

I run 295 75r16 and the tuner is set at 275 75r16 and works fine for me and pretty much is right on using my GPS.