[jgeorger]

So I've been poking around the shift points and looking at bluecat's spreadsheet. I noticed something strange: my tire diameter in HPT is set to 26.22. If you work out the numbers, a 245/40R19 is 26.72" tall. Verified that looking at the OE tire specs at tirerack.... Any idea why the half-inch discrepancy?

 

[Bo White]

Weight if the car depressing on the tire.

 

[bluegoat06]

Use the chalk method if you want to make sure, but the GM calibration is spot on if you're wearing stock wheels/tires.

 

[jgeorger]

Comparing my car to GPS running Waze app on iPhone: Car 75 mph, GPS 73 mph. This difference could be explained by the discrepancy. If I feel like testing I might change the diameter to 26.72.... Will report back if I do.

 

[whatnext]

A half inch is only 2%

 

[jgeorger]

2% of 75 is 1.5, easily within the precision of my "measurement" since I only had whole numbers to work with. Again, just something I noticed when the traffic finally allowed me to cruise at a steady 75 mph. Agree 2% is nothing to worry about, one wonders how much variability there is between tire manufacturers....

 

[bluegoat06]

Use chalk to measure the real diameter, this would account for your every day rear PSI and car weight. chalk the rear tire, do one full rotation with the car moving forward, then measure the distance between chalk marks. After that you can adjust if you want. Some tires are a tad taller, some others are shorter.