MPH = ( C * RPM ) / ( 88 * T * R )

Where:

MPH = speed in miles per hour
C = tire circumference in feet
T = tranny ratio for gear in use
R = rear end ratio

The 88 comes from 60 / 5280, the 60 bringing things from minutes to hours and the 5280 being the base distance of a mile so we get the result in miles per hour.

Tire circumference can be calculated in several ways:

1) Use wheel size and tire spec such as P75 225 15 which means the tire rides 70 percent of 225 millimeters off the wheel. Convert that to inches and add half the 15-inch wheel size to get radius and then circumference is pi * radius squared. But there is squish from the vehicle weight, so

2) I like to put the truck on flat concrete and chalk mark the tire at the bottom and on the cement, then push it forward until the chalk points down again and measure the distance. In a parking lot you can go multiple rotations for greater accuracy. Just divide the distance by the number of rotations. This accounts for the tire squish, but that could change at say 70 mph with wind perhaps giving lift to the vehicle.

3) Just take a tape measure from the hub center to the ground.

Maybe use all three and average. Regardless, the end result is approximate, but close enough and then there is speedo error too.

Have fun, but ultimately---get a tach! They are a piece of cake to hook up and a lot more fun. I recomment mounting it smack center on top the dash where it is in your face. JEGS has a nice 6K Autometer mini tach for about 65 bucks, thought the lighting on my 25$ JC Whitney 8K tach is much better.


INLINE SIX POWER! '95 F150 XL
300 Cubic Inches of Low RPM Truck Torque! And twin-I-beams too!
"Drive a stick young man! There'll be time for automatics when you're old and unable."