@Randy Dandy,
Inches in 1 mile: 63360
Minutes in 1 hour: 60
pi = 3.14159265 to 8 d.p.
Inches per minute = (mph * 63360) / 60
Inches per minute = (10 * 63360) / 60
Inches per minute = 633600 / 60
Inches per minute = 633600 / 60
Inches per minute = 10560
circumference = pi x 28 inches
circumference = 3.14159265 x 28 inches
circumference = 87.9645942 inches
rpm = inches per minute / circumference
rpm = 10560 / 87.9645942
rpm = 120.0483000