Speedometer Problems on Vintage Automobiles

Speedometer on a classic car

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Have you ever taken a look at the speedometer on your classic car and the needle was shaking in rhythm with a horrendous high-pitched sound? Fortunately, problems with mechanically operated speedometers can often be resolved with a little effort. Here we'll talk about how they work and common problems on vintage automobiles.

Gear Driven Speedometer Operation

Whether you have a 1969 Chevrolet Nova Super Sport, a 50s era Oldsmobile Rocket Eighty-Eight, or even a British sports car like a Jaguar E-Type the speedometers basically work the same. A signal originates from a driven gear that meshes with the transmission tail shaft. This set up rotates a flexible metal core inside of a speedometer cable, which in turn, connects to the back of the speedometer head mounted in the instrument cluster.

The faster the tail shaft spins the higher the reading on the dashboard. This type of implementation provides car makers with some flexibility to change calibration by changing the size of the gear that mounts on the transmission. For this reason, you often find a specific colored speedometer gear for different tire sizes and rear differential ratios. In fact, counting the number of teeth on the gear and knowing its color are helpful in diagnosing speedometer calibration issues.

Types of Speedometer Problems

One of the most annoying speedometer problems is squeaky operation. A high-pitched sound is generated by the metal core rubbing inside of the cable sheath. A speedometer head can also generate noise, which also spins along at the same speed. If you disconnect the cable from a speedometer head and it still makes noise, then you have just isolated the problem as the cable itself.

However, if the noise disappears when it's disconnected then the head contains the issue. As mentioned above, another common problem is the calibration of the reading. Sometimes owners don't find out just how far off a speedometer is until they're driving down the highway at 55 MPH and receive a speeding ticket for traveling 10 MPH over the limit.

Changing a rear differential gear ratio or the wheel and tire size are two reasons for the speedometer to provide incorrect readings. However, other upgrades like replacing a three-speed automatic transmission with a four-speed automatic or switching a three-speed manual transmission to a modern five-speed overdrive unit will also cause erratic readings.

Repairing Speedometers

When they assembled the speedometer cable at the factory, they filled the cable with bearing grease and then sealed both ends. This lubricant can leak out, deteriorate, or dry up over long periods of time. Without lubrication, the operation becomes noisy, but this is not the only problem. Since the cable runs from outside the car to up under the dash it takes a few twists and turns along the way. This can cause a binding situation that slows the cable down.

This results in a shaky needle that can make it difficult to read the miles per hour or at least drive you crazy trying to. Replacing the speedometer cable is one option, but there are two methods for lubricating the old cable that’s worth it to try first. For one method, they make a special speedometer cable lubricant that's installed where the cable attaches to the speedometer head. This specially formulated penetrating oil uses gravity to work its way down through the cable.

The second method attacks the situation from the other end. Lisle tools and the help auto parts brand make a gizmo that connects to the transmission side of the cable. It's called a speedometer cable lubrication tool. It has a Zerk fitting that connects to any standard grease gun. This allows you to pump fresh lubricant up inside the cable. This can often solve problems but is not always successful.