Speedo Gears 101
We sell a variety of speedo gears and they’re all a little different. It isn’t just the color of each one, but each one has a different amount of gear teeth. We figure out how many gear teeth will be right for your application by using a calculation of how many drive teeth are in the transmission, what the tire diameter is, and what the axle ratio is.
Here’s a calculator you can play around with; all of the transmissions we sell have 7 teeth on the drive gear and you want to calculate the driven gear:
You can see in the image above that the teeth are like stairs, each one exactly one tooth higher than the last (except for the last one). However, to get your speedometer reading exact, you may need something in between these steps, which are about 6 mph apart. US Federal law (and the laws of other countries as well) allows some wiggle room between your actual speed and the speedometer reading in your car.
If you want a very accurate reading of your speed, you can go to an electronic or GPS speedometer. You can also adjust your speedometer with a ratio adapter. A mechanical ratio adapter is a small gearbox that takes the rotation of the output from the transmission and either slows it down or speeds it up using a couple of gears inside the gearbox. This website offers tips on testing your ratio and collecting the numbers you'll need to provide to purchase a ratio adapter: Bob's Speedometer.