[sswee]

From years in the machine shop, let's see if I can clarify. The formula Caswell gives is a short cut for the calculations. The long way is to find the circumference by multiplying Pi times the diameter. Multiply the answer by the RPM running. This gives you the IPM (inches per minute). Divide this by 12" to get the SFPM (surface feet per minute).

On autos one tire size is approx. 3 mile difference on the speedo.

6" wheel at 3000 RPM's

3.14 * 6 = 18.8495" circumference
18.8495" * 3000 = 56548.6677" IPM
56548.6677" / 12 = 4712.3889' SFPM

6" / 4 = 1.5
1.5 " * 3000 = 4500

The Caswell formula is more closer to actual since when you bear into a wheel you are not running at the 6" diameter but at 4" to 5".

SS