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.
Speedometer cable assemblies...
Speedometer cable assemblies for all Mustang applications are available from Virginia Classic Mustang. The longest cables-80 inches-are for '65-'73 Mustangs with Top Loader four-speed transmissions where cable entry is on the right-hand side instead of the left. We're replacing the speedometer cable on a '68 Mustang with C4 Select-Shift; the part number is IN892, priced at $19.95 plus shipping and any appropriate sales tax.
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.
This end of the cable drives...
This end of the cable drives the speedometer. When cables fail, they typically break here. However, cables can break where binding has been the worst. When binding or seizure occurs, the cable twists up like a barber pole and breaks. The speedometer needle will bounce around like a wild child, then return to zero when the cable fails.
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.
Here's the transmission end...
Here's the transmission end of a speedometer cable, which takes a #N751 o-ring and the appropriate speedometer drive gear, chosen depending on the transmission and rear axle gear. New cables are already equipped with the o-ring. The barber pole pathway carries transmission lubrication to the drive gear.
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 Number | Nylon Teeth | Description |
| #IN1016 | 16-Tooth | Three-Speed Manual & C4 Automatic |
| #IN1017 | 17-Tooth | Three-Speed Manual & C4 Automatic |
| #IN1018 | 18-Tooth | Three-Speed Manual & C4 Automatic |
| #IN1019 | 19-Tooth | Three-Speed Manual & C4 Automatic |
| #IN1020 | 20-Tooth | Three-Speed Manual & C4 Automatic |
| #IN1021 | 21-Tooth | Three-Speed Manual & C4 Automatic |
| #IN2016 | 16-Tooth | Four-Speed Only |
| #IN2017 | 17-Tooth | Four-Speed Only |
| #IN2018 | 18-Tooth | Four-Speed Only |
| #IN2019 | 19-Tooth | Four-Speed Only |
| #IN2020 | 20-Tooth | Four-Speed Only |
| #IN2021 | 21-Tooth | Four-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.
 Before instrument cluster...  Before instrument cluster removal, protect your Mustang's steering column with a towel, which gives you the freedom to rest the cluster on your column. |  The turn signal lever is removed...  The turn signal lever is removed next to prevent switch damage and improve access. |  For speedometer cable access...  For speedometer cable access on '67-'68 Mustangs, you can remove either the climate control panel or the instrument cluster. Cluster removal makes sense if you're going to perform maintenance, such as bulb replacement. For '67, there are six screws; five for '68. |
 If you're going to remove...  If you're going to remove the cluster, climate control removal is unnecessary. When you remove the climate control bezel, speedometer cable access is easy without removing the instrument cluster. Just reach up and to the right through the opening. |  Cluster removal is straightforward....  Cluster removal is straightforward. Gently pull toward you, taking care not to damage the cluster. Pull it out just enough to reach the cable end. |  Turn the cable end counterclockwise...  Turn the cable end counterclockwise to disconnect. You may need an open end wrench. |