Jim Smart
June 30, 2006

If you've ever performed any kind of driveline or wheel and tire swap, you're probably familiar with speedometer calibration. By stepping up to larger tires and/or wheels, the speedometer reads slow compared to the actual vehicle speed. Change your rearend-gear ratio and the speedo reads high. It becomes a vicious circle as you change components and swap rolling stock. Sound familiar?

Speedometer-gear calibration and selection should be an integral part of any changes to your Mustang's chassis or driveline. Changes to wheel and tire size alter the speedometer reading because the larger tire diameter reduces the number of wheel revolutions per minute, in turn lowering the speedometer reading. When reducing the overall tire height, wheel revs are increased, which increases the speedometer reading. By the same token, a rearend-gear swap changes the speedometer reading because of the altered driveshaft speed. Speedometer gearing and calibration have to change accordingly.

Speedometer Function
Mechanical speedometers were used in '64-'93 Mustangs. They are more complex than their electronic counterparts, used in '94-and-up Mustangs, consisting of drive gears, cable, a drive head, a rotating bar magnet, a needle shell, and the odometer assembly.

Speedometers get their motivation from the transmission-tailshaft drive gear, which drives the speedometer cable. From the gear, a rotating cable drives the bar magnet, located inside a lightweight metal shell, which is tied to the speedometer needle and a watch spring, which we'll call the return spring. The needle shell is drawn to the magnetic field created by the spinning bar magnet. The return spring creates pressure against the magnetic field as the bar spins. As the bar magnet whirls around, the needle shell is attracted to the magnet. The bar magnet rotates faster with increasing vehicle speed, drawing the needle shell clockwise with its rotation. The faster the magnet spins, the higher the needle moves around the dial. As vehicle speed decreases, magnet speed decreases, causing the needle to swing back toward zero with help from the return spring. The spinning magnet is always counteracting the return spring that's trying to bring the needle back to zero.

Odometer function is a matter of gearing from the speedometer head. A series of gears originating at the head drive the odometer. Each of the number wheels in the odometer has cogs that move neighboring wheels along as mileage progresses. A mile moves each time a tenth completes a full 360-degree cycle. It becomes a chain-reaction increase as each wheel makes a full 360-degree spin.

Speedometer calibration involves the right combination of tires, wheels, and speedometer drive gears. But there's more to it if you want pinpoint accuracy. Take your lead from Ford's old Master Parts Catalog (available on CD), which tells what combination of gears to use based on factory-tire sizing. We're going to help you get started with the basics of speedometer-gear selection and how to fine tune.

For one thing, get used to the idea that you're never going to achieve perfect speedometer calibration. It doesn't even happen in police cars despite the best efforts to keep them calibrated. Your speedometer will always be off to some degree because there are endless variations that cause inaccuracy. For example, temperature and humidity effect speedometer accuracy because they change the return-spring tension. Tire irregularities also cause inaccuracies. A dip in the road can cause a brief inaccuracy. Entering a turn can momentarily cause an inaccurate reading because we're messing with tire diameter. As tires wear, speedometer accuracy changes. Install new tires, and it changes again. A cold tire is smaller than a warm tire.

Speedometer accuracy is determined by a road test alongside a vehicle with an accurate speedometer. If one isn't available, a stretch of interstate highway with mile markers works best. Begin your speedometer check at a mile marker and a given tenth. If the odometer comes up short at the next mile marker, your speedometer is geared too slow. If your odometer winds up beyond the tenth you began with, the speedometer is geared too fast.

A speedometer geared too slow needs a driven gear with fewer teeth. By the same token, a speedometer geared too fast needs a driven gear with more teeth. Calibration should begin with the factory-suggested gearing for your Mustang; fine-tune it from there.

Choosing the Right Gear
Calibrating a speedometer is like bodywork and paint: To do a good job, you must have proper preparation. In this case, it starts with the correct speedometer drive gears. Without the correct gears, you can't fine-tune speedometer performance.

Ford's Master Parts Catalog provides the right gear combination for factory-original applications based on tire size and axle ratio. There are two basic part numbers for the speedometer drive and driven gears; PN 17285 for the drive gear on the transmission tailshaft and PN 17271 for the driven gear on the speedometer cable.

There are eight types of Ford drive gears located on the transmission tailshaft, but only two apply to Mustangs; Type 3, which is a steel gear, and Type 4A, which is nylon in either pink or black. Six other types were used throughout the Ford car and truck line.

Three basic types of driven gears, located at the transmission end of the speedometer cable, apply to Mustangs; Type 3, Type 3a, and Type 1. Type 3 has a short pilot shaft, and Type 3a has a long pilot shaft. Type 1 is a different type of driven gear altogether because it only works with a special adaptor found on Mustangs with 3.91:1, 4.11:1, and 4.30:1 axle ratios.

When shopping for speedometer-drive gears, it's important to remember left-side or right-side because this determines proper gear-tooth angle. Left-hand or right-hand depends on transmission type. All C4, C6, FMX, AOD, AODE, and three-speed manual transmissions have speedometer cable access from the left-hand side. Ditto for the rare Dagenham four-speed. Both Ford Top Loader and Borg-Warner T10 four-speeds have access from the right-hand side. This means gear-tooth angle is different from one side to the other. It must be compatible between drive and driven gears.