[Skunk Works 2SS]

Hey guys, just wondering if I can get some assistance in changing the tire sizing for my car?

I'm currently running 305/30/20s on an 11 inch wheel for my rear setup which should be a 27.2 inch tire.... I went into HP Tuners Gear/Tire wizard after calculating the size and input 27.2 in tire size and stock gear ratio. I left the speedo calibration and scale trans parameters boxes checked with a 1.0 multiplier, saved the changes and wrote the calibration but my speedo was still 2-3mph off. So I went back in and upped the tire size to 28.2, saved, wrote, same issue. Still 2-3mph off what my GPS speed is saying on both my phone and radar detector.

I'm just wondering if there's something I missing here as to why the tire size change isn't bringing my speedo reading into line with my GPS speed? Do I need to perform a write entire??

Any help on this would be much appreciated and I thank you in advance!

[wnta1ss]

The place in HPT where I've made a speedometer correction successfully is the Speedo tab, Calibration, VSS tire revs per mile. Have not tried this wizard that you are talking about. Write calibration worked fine for me with a change to this parameter.

[Silverwing]

What GPS system are you using to detect speed?

AFAIK most, if not all, GPS systems have an issue being 100% accurate on speed due to the way GPS works. There's a bit of delay with GPS and that delay can cause problems if you're using it to detect speed.

[Skunk Works 2SS]

Originally Posted by Silverwing

What GPS system are you using to detect speed?

AFAIK most, if not all, GPS systems have an issue being 100% accurate on speed due to the way GPS works. There's a bit of delay with GPS and that delay can cause problems if you're using it to detect speed.

I'm using both my Passport 9500ix radar detector and my phone with a GPS speed app to determine true speed. On the highway I would put it in cruise to keep as close to a constant speed as possible and then compare the vehicle to my GPS. With stock tires, all 3 matched perfectly at 60mph, 70mph, and 80mph. After going to the 305/30/20, my speed as reported by my car was now 2-3mph slower than GPS speed even after setting tire size to 27.2.

I did play around in HPT some more than and decided to go down in size rather than up. It seems setting the tune to a size of 26.4 has me back to being synced with GPS speed again. I'm not sure why I need to set the size almost a full inch smaller than what I'm actually running to get the reported speed correct, but as long as it's reading properly again, I'm happy.

[Brianw36]

After making change to tire wizard do you do a full write of ecm and tcm?

[Skunk Works 2SS]

No. Just write the calibration.

[Brianw36]

Originally Posted by Skunk Works 2SS

No. Just write the calibration.

Do I select both ecm and tcm?

[Skunk Works 2SS]

ECM only

[Brianw36]

Thanks