You have to figure out what size tires you are running. Once you have that, you need to know the circumference. Many tires have specs you can look up on line. The number you are looking for is revolutions per mile. Once you have that number it is easy to translate that to RPM at 60mph. in drive. Multiply that by the overdrive ratio, .i.e. .87 x rpm@60 in 1:1 = overdrive rpm.
Steve
