If any of the following have changed over the life of the car, the speedometer reading will be affected:
1) Rear axle ratio
2) Tire size
3) Speedometer gear in the transmission.
If you are sure none of these has been changed, then the problem is most likely in the speedometer itself. The speedometer has a rotating magnet which works against a spring. If the spring has weakened or has broken loose, the reading will be high. You would need to either find a replacement speedometer, or send yours to a shop specializing in instrument repair.
I own a 67 also. Does the speedometer read 30mph when you are stopped? If so, get your speedometer rebuilt, or replace it with one from an aftermarket company.
If the problem is exponential (the gap between actual speed and displayed speed gets greater the faster you go) then the problem is the speedo gear in the trans. If you remove the single bolt that holds the speedo cable in the trans, and pull the cable out, there is a small plastic gear on the end of the cable that is held to the cable by a small C-clip. This will have to be replaced with the appropriate gear. Most mechanics have a chart that has which gear is right for which rear axle gears and which size tires, as well as the transmission the car has.
i have calibrated many , many mustangs. here is what's up.
first off, there is nothing wrong with your speedometer. the calibration is done at the transmission.
There is a little gear on the end of the speedometer cable and housing at the transmission. The gear has a range of 15 to 21 teeth. For every one tooth you go up or down: you offset the speedometer by 3 miles per hour.
so you will need to pull the gear and count the teeth. then go down to ford and order the largest gear for your vehicle.
If the gear can not calibrate the vehicle; then a ratio box will need to be made for the car. A ratio box is a device that slows or speeds up the rotation of the cable.
If i calibrate your mustang with a gear; i would charge you $90 and it would take about 30 minutes.
If i calibrate your mustang with a ratio box; i would charge you about $150.00 and it would take me 90 minutes.
If you take it to a transmission shop they can either re-gear the tail shaft internal gears or put on an external gear reducer between the speedo cable and tranny tail shaft. Tires change mph, so does changing tranny and rear end gears.