CTS-V (Auto) Correct HPT Tire Size Value for RE-11 305/30-19?

[bbradyc5]

Anyone here with an auto CTS-V who runs Bridgestone RE-11's in 305/30-19 size calculate or figure out correct tire size value for HPT? Stock tune tire size value is 26.05" dia where Michelin specs the tire dia at 26.9", so I assume lower profile 305/30-19 RE-11 with mfg spec dia of 26.3" should be entered as some value less than 26.05" in tune. Short of doing rollout measurement on factory tire size (285/35-19) and on new lower profile 305/30-19 to see how it compares to factory tune value, I figured I'd check to see if someone already figured this out.

[04blackgmc]

Use 26.3....

[bbradyc5]

OK I'll be the first to admit I am a noob at this so my logic says the value should be less than the factory tune value of 26.05" if that was for a 26.9" spec'd factory tire...care to elaborate on why my rationale is wrong?

[04blackgmc]

I've always used the Manufacturer tire spec for tire size for the Tire calc portion of the tune.

[8850]

You want the VSS tire revs. per Dist to get smaller with a larger tire and larger value with smaller tire dia. Use a GPS and change tire diameter accordingly.

[2xLS1]

Enter the stock tire size and the new tire size here. http://www.discounttire.com/dtcs/infoTireMath.dos

Take the +/- revolutions per mile and +/- the value from the stock tire revs per mile in the calibration.