Calibrate Speedometer for New Rear-End Gears

Q: How to I calibrate my speedometer for new rear-end gears?

A: Determining the precise speedometer drive gear/driven gear combination required for a rear axle ratio change is surprisingly easy-if you know the existing combination and axle ratio. Whip out your calculator and follow along. Let’s assume that your F-Body is currently equipped with 2.73 gears and that you’re planning to install a 3.73 ring and pinion. If you didn’t change speedometer gears, actual vehicle speed would 73.1 percent of indicated speed (2.73/3.73 = .731). That being the case, what’s required is speedometer gearing that will result in the driven gear spinning at 73.1 percent of its current speed. The speedo gear combination for a 2.73 rear (assuming 26-inch diameter tires) is a 17-tooth drive gear and 37-tooth driven gear. Switching to a 45-tooth driven gear (the most teeth available) would translate to the vehicle traveling at 82 percent of indicated speed – still a 10 percent error.

Obviously, the drive gear must be changed to one with 15 teeth if the 73.1 percent ratio is to be achieved. Going back to the original 37-tooth driven gear for illustration purposes, the 15-tooth drive gear would result in the vehicle traveling at 88 percent of indicated speed – that seems like a step in the wrong direction, until the driven gear is changed. (This is where it gets a little tricky because you have to work with a percentage of a percentage). Matching the 15-tooth drive speedometer gear up with a 44-tooth driven gear brings the drive ratio to 74 percent (15/17=.88; 37/44=.84; .88 x .84 = .739, which rounds to .74, or 74 percent). Still not quite enough. Will a 45-tooth driven gear do the job? Run the numbers through your calculator and you should come up with 72 percent. That’s about as close as you’re going to get.

In case you haven’t put all the pieces together, there is a distinct limitation regarding speedometer accuracy and steep rear gearing. Unless driven gears with more than 45 teeth, and/or drive gears with fewer than 15 teeth become available, it will be impossible to have an accurate speedometer with rearend gear ratios lower (higher numerically) than 3.75:1 – unless 27-inch or larger tires are installed.