[killakev]

My 68 speedometer is making noise as I drive the car. The cable is brand new and lubed so I figure the mechanism in the speedo needs attention. I would describe it as a clicking sound, the needle does slightly bounce. I have read other threads and people discuss the cable but like I said the cable is brand new.

Anyone experience this problem and ideas how to fix it?

[Peter Serio]

After 52 years the oil in the bushing that is part of the speedometer is all dried out. It is an oilite bushing. The bushing should be replaced along with the magnet and shaft assembly that rotates inside of that bushing.

The jumping pointer is a sign that the speedometer is about to lock up. It is binding due to lack of oil, plus once you get the magnet out the shaft is always covered with what looks like black dust or gummy grey powder. It is the end result of dried up oil mixed with tiny powdered bits of the bushing copper & brass which is started to fall apart.

All old cable driven speedometers need maintenance after 40 years, that or if the car has sat for a long time un-driven.

The old oil dries out after around 40 years. Best to have that speedometer overhauled before it locks up.

There are a bunch of special tools required to do that job. Plus a 24 volt DC magnetizer and a 120 volt AC de-magnetizer.

The strength of that grey iron magnet on the end of the rotating shaft is how those old speedometers are calibrated.

You have to have a way to magnetize the iron and then carefully remove a small amount of that strength a little at a time to get the pointer to read true. It can take time to get one to read just right.

[north]

I’ve noticed something interesting in my collection of 69’s.... all my full-size cars have speedos that read very accurately (within 1mph at 60). But all my A and F bodies all read 5-8% high. All have original drivetrains and correct diameter tires. Maybe it’s just a coincidence, maybe the sportier models were set to read a bit fast or maybe the B body speedo unit holds its precision better than the AFG bodies shared round speedo units.

[Red Box Rebel]

Originally Posted by north

I’ve noticed something interesting in my collection of 69’s.... all my full-size cars have speedos that read very accurately (within 1mph at 60). But all my A and F bodies all read 5-8% high. All have original drivetrains and correct diameter tires. Maybe it’s just a coincidence, maybe the sportier models were set to read a bit fast or maybe the B body speedo unit holds its precision better than the AFG bodies shared round speedo units.

My 68 Original GTO Speedometer has always read right at 5% too high for both speed and actual mileage. Owned the car sine 1974.

[68gtoMN]

Why would one then not order a speedometer gear that is lower by that percentage for your cars? It would make reading more accurate AND not log too many miles on car. I'm looking into doing this very thing now

Speedometer on my Ford Flex ecoboost is SPOT ON with radar at 60. Driving next to my 68 GTO Convertible AT with 3.36 rear, the GTO reads 65 while Flex shows 60.. That's a 9.2% difference on the high side using stock wheels/tire sizes. Swapping for a 3.08 gear would represent a 9.23% decline in reading compared to 3.36.

Other examples:

• Car with 3.55 gears reading 5% too high -swap for 3.36 speedometer gear, which is 5.35% lower
• Car with 3.36 gear reading 5% too high - swap for 3.23 speedometer gear, with is 3.9% (4%) lower

It seems logical but perhaps I"m missing something. Is my thinking wrong?

[north]

The driven gears on most of our cars have give or take 20 teeth so for each extra tooth will lower the speedo reading by about 5%.

The driven gears are easy to replace. If to achieve the right ratio you need to replace the drive gear (the ring gear on the output shaft) it’s a bigger job as you’ll have to open the transmission.