What rear axle gears are you running? You may have the wrong speedo driven gear on the speedometer pickup.
Speedo Gear selection copied from Ford Racing Performance Products
Speedometer cable replacement for 87-93 Mustangs
Revised 11 Mar-2015 to add tip on what to look for when there is a bouncing speedometer.
How the speedometer works:
The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk. There is very little clearance between the disk and drum, only a few thousands of an inch.
Lubrication warning
Use a graphite based lubricant for the speedo cable. It is available at most auto parts stores in a very small tube. Lubricate only the lower half of the cable. The reason for this is that if you use too much lubricant, it works its way up into the speedo head and gets between the rotating magnet and the disk. This causes the speedo to seize up and may wring the indicator needle off the indicator spindle. You may be able to fix things up with non-flammable brake parts cleaner to clean the disk and magnet assembly. Count on replacing the current cable and housing with a new cable and housing. You need to do this to prevent the excess lubricant from causing it to fail again.
A speedometer that bounces around is either a cable problem (top cause) or a speedometer drive gear that is chewed up. When you remove the cable drive end, check the teeth on the driven gear (the one attached to the cable) or the drive gear (the one on the transmission output shaft) Late Model Restoration (
http://www.latemodelrestoration.com/products/79-93-Mustang-Transmission-Speedometer-Correction) has a complete range of replacement speedometer gear sets.
Speedometer cable replacement .
Note: All 89-93 cars have a VSS sensor even if they do not have cruise control. The 87-88 only have a VSS sensor if they have cruise control.
Speedo cable housing assembly without cruise control:
The VSS equipped cars have a speedo cable with a different fitting on the transmission end of the cable. It is the fitting on the LH side of the following picture.
Speedo cable housing assembly with cruise control
Preparation: if you are only going to replace the inner part of the speedo cable, get lots of newspaper or a painter’s drop cloth to cover the inside front of the car. About the time you have the dirty, oily speedo cable core all over your lap and the inside of the car, you will thank me for this suggestion.
Replacing only the inner cable: see steps 1- 6, 12, 13, 17-21
Replacing the housing and inner cable as an assembly: see steps 1-11, 13-21
Inside the car:
1.) Remove the shield around the steering column that covers the ignition switch & turn indicator switch.
2.) You now have access to the two screws that hold the lower part of the cluster housing in place. Remove them and place them in a zip bag.
3.) Use a stubby or an offset Philips screwdriver to remove the two screws on the top of the cluster housing. The screws are up close to the windshield, so they can be hard to get at.
4.) The cluster housing will now slide forward: depending on your particular car, you may or may not have to disconnect the wiring for the headlights, hazard lights, or cluster wiring. All of the wiring uses plastic connectors with plastic spring clips on them. To release the connectors, lift the plastic clips and pull straight back.
5.) The speedo cable is secured in the speedo head by a white plastic clip. Depress the clip or squeeze it and pull the cable out of the speedo head. This can be tricky, but it will come out if you have the white clip depressed enough.
Speedo head cable clip
Photos courtesy of Almost Stock
6.) With speedo cable removed from the speedo head, try twisting the cable end with your fingers. If it turns more than 1/4 turn, the cable may either be broken or you have damage at the other end where the cable mates to the VSS sensor or speedo pickup gear in the transmission.
Outside the car, replacing the cable housing assembly.
The following steps are necessary only if you plan on replacing the cable & cable housing assembly.
7.) If you are going to the replace the cable housing, the next step is important. Tie a study string or wire to the VSS sensor end of the cable housing. This string or wire is to be used to fish the cable housing back through the maze of wires that is under the dash. If all you are going to do is replace the inner cable, you can omit this step.
8.) Jack up the car, all 4 tires must be off the ground. Place jackstands under the car for safety.
9.) Locate the VSS sensor on the driver’s side of the transmission tailshaft housing. The speedo cable housing will be secured in the VSS sensor with a hairpin clip
Do not remove the clip!!!: The hairpin clip stays in place. If you remove it, the odds are that you will not be able to get the cable to stay in place on re-installation. Pull firmly straight back on the cable housing and it will come out. A considerable amount of effort may be required to get the cable out of the VSS sensor, but it will pull out.
10.) Release the cable housing from the clips that secure it to the car body.
Inside the car:
11.) If all you are going to do is replace the inner cable, you can omit this step.
The housing assembly can then be pulled out and the fish string or wire can be removed from the old cable housing and secured to the new one.
12.) You can omit this step if you are replacing the cable & cable housing assembly.
The inner cable can be removed by pulling it out of the housing assembly. Watch out for the lubricant so that you don’t get it on the car’s interior.
13.) You can omit this step if you are replacing the cable & cable housing assembly.
Lubricate only the lower part of the new cable with speedometer lubricant or graphite. Don’t use too much lubricant, or it will work its way up into the speedo head unit and damage it. Thread the inner cable into the housing, turning it as you go. When you are all the way in with the new cable, it will engage the VSS sensor and stop turning.
Outside the car, replacing the cable housing assembly.
The following steps are necessary only if you plan on replacing the cable & cable housing assembly.
14.) Use the fish string or wire to feed the cable housing assembly through the dash wiring and out the cable hole in the firewall.
15.) Secure the cable in the body clips, making sure that the cable isn’t rubbing against the exhaust pipe.
16.) Push the cable housing assembly into the VSS sensor until it snaps in past the hairpin clip. Connect VSS wiring connector back to VSS sensor.
Inside the car:
17.) Push the cable housing back into the speedo head unit. You should be able to feel the white clip click into place.
18.) Reconnect all the wires & connectors on the speedo head unit.
19.) Re-install the cluster unit in the dash & tighten the 4 screws that hold it in place.
20.) Re-install the cover for the ignition switch & turn signal.
21.) If the car is up on jackstands, start the car, place it in gear & watch the speedo to see if it works OK. If you didn’t jack the car up, take a test drive.
