Jim Smart
July 21, 2010

In the May issue, we addressed classic Mustang instrumentation from an electrical system standpoint-temperature, fuel, and oil pressure gauges along with ammeters. But what about the nuts and bolts of the old, time-proven speedometer?

From '65-'93, Mustang speedometers were cable-driven mechanical instruments that worked on the eddy current principal-a magnet whirling around inside a shell attached to an indicator needle. The faster the magnet whirls around inside the shell, the higher the needle goes against mild spring pressure that wants to return the needle to zero. The result is a reading we hope is close to the speed of our Mustang.

Speedometers do wear out over time and mileage, which calls for a professional who knows how to repair them. We will get into that shortly. But what about things you can do yourself? If you're wrestling with a bouncing speedometer needle or one that's completely inoperative, the problem may be your speedometer cable.

Speedometer needles bounce when there's cable bind or damaged drive gears. When do you replace the cable inside and when do you replace the entire cable assembly? Much depends on cable assembly condition.

Cable function depends hugely on lubrication and plenty of it. When lubrication becomes lean, cables bind and deteriorate, which only makes the problem worse. If the cable has deteriorated to where it is frayed anywhere along its length, it's time to replace the entire cable assembly. Frayed speedometer cables damage the inside of the sheath to create two sources of binding. This is why entire cable assembly replacement is so important to smooth speedometer operation.

When you install a new speedometer cable or service an old one, generously lubricate the cable with a mixture of transmission fluid and lightweight lithium grease. Mix up a modest wad of lithium grease and transmission fluid in the palm of your hand and run the cable through it. You can also use speedometer cable lubricant for this purpose. Run the cable back and forth until saturated with lube. Then, slip it into the cable sheath. It will serve you well for thousands of miles and many years.

Virginia Classic Mustang
Guide to Speedometer Drive Gears

Part NumberNylon TeethDescription
#IN101616-ToothThree-Speed Manual & C4 Automatic
#IN101717-ToothThree-Speed Manual & C4 Automatic
#IN101818-ToothThree-Speed Manual & C4 Automatic
#IN101919-ToothThree-Speed Manual & C4 Automatic
#IN102020-ToothThree-Speed Manual & C4 Automatic
#IN102121-ToothThree-Speed Manual & C4 Automatic
#IN201616-ToothFour-Speed Only
#IN201717-ToothFour-Speed Only
#IN201818-ToothFour-Speed Only
#IN201919-ToothFour-Speed Only
#IN202020-ToothFour-Speed Only
#IN202121-ToothFour-Speed Only

At press time, all speedometer drive gears were priced at $6.95 each. If your speedometer reads low, select a drive gear with fewer teeth. If it reads high, choose a drive gear with more teeth. If it reads too high and you have an 18-tooth drive gear, opt for a 17-tooth. By the same token, if it reads too high and you have an 18-tooth, swap to a 19-tooth and see what happens to speedometer indication. Instead of a gear swap, a good speedometer shop can calibrate your speedometer.

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