Hypertech Speedometer Calibration

dcarl · January 14, 2015

I'm sure this has been answered but I can't find it. I have a 2015 Sierra SLT that I just put a set of Toyo Open Country AT2 275/60R20 on and I bought the Hypertech Speedometer Calibrator part number 732501, entered 33" as the tire diameter and now my speedometer is reading 2+ mph faster than actual as measured by a GPS. Based on normal calculations, it should be the other way around (reading slower).

I even went back and reflashed it just to make sure, and still the same. My question is I've gone through the Owner's manual and I don't see a specific fuse for OnStar. During the calibration procedure, the DIC came on and displayed Bluetooth connections, and other normal information. I'm wondering if and what fuses I need to pull to disable certain systems.

Any help here would be greatly appreciated.

Thanks!

BuckNaked · January 16, 2015

Measure actual tire height with and put that in the programmer, don't go by what the manufacturer says.

Hypertech · January 16, 2015

Yes do what BuckNaked said to do and it will correct for your actual tire height

redwngr · January 16, 2015

Better yet, look up the tire dimension on the tire manufacturer's website or one of the tire distributor sites.

BuckNaked · January 16, 2015

Tire manufacturers will tell you the tire height while they're not installed on a vehicle. You have to measure the height while tires are on the vehicle with the weight of the vehicle on them to get the correct height to put in the programmer. That will correct the speedometer.

dcarl · January 17, 2015

Thanks everybody. I called Hypertech, and the guy told me to put a straight edge on the top of the tire and measure from the ground up. It turns out my "Advertised 32" on the Toyo Open Country AT2 275/60R20 is really only 32 1/16" when inflated to 32 psi. So I ended up using 32" as the tire size in the re-calibration and all is good now.

Don't understand why they don't put that in the instructions. Also makes me curious about the revs per mile data the tire manufactures publish...

second2none · August 23, 2015

Thanks everybody. I called Hypertech, and the guy told me to put a straight edge on the top of the tire and measure from the ground up. It turns out my "Advertised 32" on the Toyo Open Country AT2 275/60R20 is really only 32 1/16" when inflated to 32 psi. So I ended up using 32" as the tire size in the re-calibration and all is good now.

Don't understand why they don't put that in the instructions. Also makes me curious about the revs per mile data the tire manufactures publish...

Did you pull the fuses that it says to pull in the instructions?.....Just curious because I am about to use this same product to calibrate mine.