You are absolutely right Bob, that ain't the way racers do it. They use a tape measure, exactly as you said. But they are solving a different problem. Racers are concerned with the circumference of the tire to be exactly the same so the car goes straight down the dragstrip, or for the circumference to be what's required to provide the correct amount of stagger to help the car turn in circle track racing. They are using those measurements to compare one tire to another, not to determine an absolute circumference value. They are looking for the difference between the two tires.
The original post was not concerned with making the car go straight or turn better, he was looking for a way to accurately predict engine RPM at a given speed. I think the suggestion I posted for that problem will get him in the ballpark.
By the way, marking the tire and rolling the car 10 revolutions and then doing the math will result in a number almost identical to the racers method of wrapping a tape around the tire. What changes, is with the car at speed, the circumference can grow slightly, which is why the tire manufacturers post a "revolutions per mile" spec at a stated speed.
