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Originally Posted by MilesPerTank View Post
I'm not running OEM tires so I will probably do that calibration with a GPS unit.
Here's another way to do it. Google Earth has a nice Ruler feature. You can use it to find the exact distance between arbitrary landmarks. It seems to offer a lot of precision and accuracy. For example, I can use it to determine that I live on a street that's 268.01 inches wide. Or that my driveway is 10,890.26 inches (0.17 miles) from the corner.
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Both these methods *might* be valid if you do them on a long, dead straight road that is dead flat. Any turns or any hills up or down are going mess up the results, because you have traveled more surface miles than the "as the crow flies" miles that google/gps are showing. The google earth ruler feature may offer a lot of digits, but I would suggest that there is no way it is good to 1/100 of an inch. You cannot even resolve where the sloped edge of a curb, or the edge of a road starts from those views.
What is the accuracy of GPS now, +/- 30 feet? So that is 60 feet uncertainty at your start and your end position. That is 1.1% uncertainty if you test over one mile. .11% if you test over 10 miles, but you had better be driving in Kansas to get a flat straight road 10 miles long.