What about the speedometer? Is it accurate? That depends upon a few variables, such as the rear axle ratio, the tire diameter, the drive gear size, and the driven gear size.

Let’s st art with the easy ones, the axle ratio and tire diameter. To find the axle ratio jack up the rear of the car and turn both rear wheels the same direction one full turn while counting the drive shaft revolutions as the wheels turn. In this case I know my rear end ratio is 3.42.

To determine the tire diameter I sit the car back on the ground and measure from the floor to the center of the rear axle. That measurement is 13.5-inches. Multiply that times two and I have the accurate diameter of the tire with the full weight of the car on it. 27-inches.

Next I need to know which drive and driven gears are in my GM T350 transmission. To do that I remove the speedometer cable and the speedometer housing from the transmission. The cable is screwed into place, the housing is held in place using a retainer.

With the speedometer housing removed I can see the speedometer drive gear mounted on the transmission tail shaft inside the unit. Mine is red. The pic is a borrowed cut-away view for demonstration purposes.

GM uses four different colors of drive gears - Gray = 15 teeth, Red = 17 teeth, Blue = 18 teeth, and Yellow = 19 teeth.

The driven gear will come out of the transmission with the speedometer housing. Mine is green which means is has 42 teeth.

GM uses twelve different driven gears. 34 = Light Green, 35 = Orange , 36 = White, 37 = Red, 38 = Blue, 39 = Brown, 40 = Black, 41 = Yellow, 42 = Green, 43 = Purple, 44 = Dark Gray, 45 = Light Blue.

Armed with this information I can now determine if my speedometer is accurate or if I need to change to a different driven gear size. Here is the formula.

1001 = Driven gear teeth

Tire revolutions per mile = 20168/Tire diameter

You can do the math if you want, but my calculations come out to equal the need for a 43 tooth gear, or in this case a Purple driven gear.

You can also go to the web and google up a speedometer calibration program that will do the math for you.

Project car provided by:

Larry Lyles, owner

LPL Body Works, LLC

Amarillo , TX 79109

www.lplbodyworks.com