Originally Posted by
JTGraphics
Best to just measure your actual wheel.
Pump it up to pressure mark the wheel or use the valve stem mark your starting spot roll one revolution and measure it, or roll 3 revolutions and divide it by 3 and use that it will be even more accurate.
There are two difficulties with roll out tests.
1) A loaded wheel will deform and decrease the effective radius at the contact point. This effectively reduces the circumference of the wheel. If you do an unloaded test, your number will be two high.
2) You have to roll a very straight line. Any curve will result in a number that is too low.
The "toothpaste" trick helps a lot, especially if you have a pitched driveway. Higher speed will get you straighter. Take the last to visible toothpaste splotches. And measure. Repeat the test 3 or more times and average the results.
My new method is to get out a GPS and just compare my computer readings to GPS readings and make changes till I get them to agree to the tenth of a mile. GPS speed is VERY accurate so long as you are going in a straight line.
For the OP with OCD. Realize that ANY method is going to be inaccurate. Changes in temperature will very pressure in the tire which will effectively increase the circumference of the wheel. The loading of a wheel will vary during your trip. Even GPS logging will under-report distance if you are on a curvy route as it's method is to periodically sample and measure distance between points.
Ultimately what is important is to compare performance from one day to another. As long as you keep the setting the same for the wheel, you can be sure you are comparing apples to apples.