My 700x23 Tri comps on the wheel roll out at 125 psi back, and 115 psi front is only 2083. I have measure it a number of times and all I get is 82 inches or 2083. My garmin handheld runners forerunner I also wear is pretty close. On a 30 mile ride some times I get about 30.1 on the Garmin and 30 on bike computer. On my old tires I used the 2096 did not use a roll out test but the Garmin was almost exact to the computer. I did a 50 miler with 50.00 and 50.02

For my running the Garmin will vary a few feet in over 10 miles no more than 50 feet. Someone told me a bike computer is more accurate and they this person has better bike credentials than me. I also find my max speed on the computer is always about 1 mph more than the Garmin. Today my Garmin had my max speed at 35 and bike computer had it at 36.1. The Garmin does not respond immediately and lags just a bit so this does not surprise me. For running it will vary quite a bit during the run at the same pace but over all almost dead accurate.

My guess is the it is pretty accurate much better than anything else and certainly a car odometer.