You can always drive a measured mile and have someone walk alongside and count the revolutions! This is the most accurate for a specific setup!
Your 285/75R15's will revolve about 633.93 times per mile. I don't know by how much this changes do to tire pressure. What you can do is. Fill your tires to the pressure you want, then on a flat no-slip surface. Mark each tire where it contacts the road, and mark a line on the road then drive very slowly, no slipping, and have someone walk along side and count 100 revolutions (about 833 feet), then mark the road again. The distance between the marks is the distance you go in 100 revolutions. (or you could modify this process and do only 10 revolutions about 83 feet).
If you measure this distance in feet, divide this measurement into 5,280 (number of feet in a mile). Then multiply by 100 this would be your number of revolutions per mile for your setup. Your tires are about 99.9484 inches in circumference, depending on inflation.
I am going to do a process similar to this ... I want to know exactly for my setup the best way is this type of measurement, must ensure no slipping, can always x-check against the theoretical value, found from the math, but a real world measurement is sometimes revealing.
what he is saying is adjust your speedo until it matches your GPS exactly. then your speedo and odometer will read correctly. go with the theoretical value then make minor adjustments from there to get it spot-on with GPS. Quicker and easier than putting marks on the street...
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