[1967SC]

Speedometer is not reading accurate speed,reads slower than actual speed.
Is this a cable problem or speedo gear problem

1967 Sport Coupe w/Criuse Control

Thanks in advance.

[ponchjoe]

Your signature states 48k original miles, any changes to the drive train or tire size? Did it just start out of the blue?

[1967SC]

Originally Posted by ponchjoe

Your signature states 48k original miles, any changes to the drive train or tire size? Did it just start out of the blue?

Just started out of the blue,still retains 14" wheel size..also still retains original rear gear.

[67gtospud]

Besides the 14" rim, has the tire size changed from original size?

[Peter Serio]

Over time the magnetism slowly diminishes, when the speedometer was made brand new GM over-magnetized them on purpose. And for about the first 6 months or a year they will read a bit more on your dash dial than actual road speed. After about a year the "set" of the iron settles into a very stable mode and it will say like that for at least the next 25-30 years. After 30 years or so, in many cases the magnetism inside of the speedometer weakens resulting in readings on your dash dial less than actual road speed. (This is assuming that all of the other parts: the cable, gearing, tire size, etc, are correct for the car.)

Every single one of these old speedometers should be serviced at least once every 30 years. The old lubrication dries out and if you run one until it "locks up" the delicate inner workings of the speedometer are destroyed.
At least once every 30 years you should have a complete tear down; through cleaning checking all the gears, the bronze bushing, the main input shaft as well as the odometer copper shaft. Then re-lubrication, re-mag and calibration.

If one locks up on you while you are driving the parts inside which will break are no longer available as service or repair parts. In order to fix one of those you need to find another complete good used undamaged speedometer to get parts.