Description

Main Speedometer Page: Click Here

To determine your tires true diameter in relation to determining what speedometer gears you need, measure from the ground up to the center of the axle then multiply by two. The actual run out of the tire can be as much as 2 to 3 inches less than what the circumference of the tire measures. In effect your tire has two diameters.

To determine your rear end ratio, raise both tires off of the ground, turn both tires at the same time one complete turn, at the same time count the turns of the drive shaft, this is your ratio.

Speedometer Calibration:
Variables such as the vehicle’s axle ratio and tire diameter affect the accuracy of the speedometer. The following speedometer drive gears and driven gears are available for the 700R4 / 4L60 transmissions. Most cars and trucks with low rear end ratios will take a 15 tooth drive gear. There are two driven gear housings, one for 34 through 39 tooth driven gears and one for 40 through 45 tooth driven gears. Look at the bottom of the speedometer face for the numbers 940, 960, 1000, or 1020. These numbers are the number of revolutions the speedometer cable must turn in one mile of travel. Your speedometer may or may not show a number. Most speedometer from 1965 on use 1000.

Twelve speedometer driven gears are available for the 700R4 / 4L60 transmissions to calibrate the speedometer with various axle ratios and tire diameters. The number on the driven gear housing must match the number of teeth on the driven gear. Unless driven gears with more than 45 teeth and / or drive gears with fewer than 15 teeth become available, it will be impossible without using a ratio adapter to have an accurate speedometer with rear end gear ratios lower (higher numerically) than 4.10:1, unless 27 inch or larger tires are installed.