Speedometer Gear Calculation

Speedo Revs/mile = (20168 x DiffGear x DriveGearTeeth) divided by (Tire-diameter x Driven-GearTeeth)

Speedometer Calibration

How to determine what transmission speedometer gear is correct and what information is needed in order to properly calibrate a speedometer.
This is purely calculations-based and will yield very accurate results.

One important piece of information which is required is the number of revolutions it takes your speedometer to register one mile.
On most modern cars, this may be found on the bottom (or elsewhere) of the speedometer face, usually in very small numbers. Look for something like 940, 960, 1000, or 1020, or something close. This number is the number of speedometer cable revolutions (or equivalent in the case of electronic sender units) to increment the odometer one mile. Most speedometers (modern ones) are 1000 revs/mile, and this is a good starting number to use.

Another vital piece of info is the speedometer drive/driven gear ratios, the gears inside of the transmission or transaxle. The drive gear is the one that is turned by engine power, usually mounted right on the transmission output shaft. The driven gear is the one that is “driven” by the “drive” gear and is connected to the speedometer cable or electronic sender. The numbers entered in the program are the integer number of teeth – just count ’em up.

This will help you compute the best-driven gear given a particular drive gear, or the best drive gear given the driven gear value.
Remember that there are only set combinations available, which you can find out from the parts store or dealer.
Also, remember that gear tooth numbers are integer values.

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.

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.