1953 Super 88 with Rocket V8 - incorrect speed indication

[P6tu]

Hello,

 

So I've aquired a "new" car to myself, but this has a strange issue. At first the speedometer wasn't working at all. The speed wire was broken from the transmission side. Bought a new one and before installing it, I found the root cause, why the wire was broken in first place - the speedometer was stuck as hell. So I cleaned it and lubricated it properly and cleaned it up as much as possible without taking it apart. It started working perfectly, but the speed it is indicating now, is not correct. It is always showing like a +30%. Driving around 40mph according to the speedo is actually 30mph and driving around 70mph according to the speedo is actually 55mph. I've read somewhere that there may be something done with the gears inside the tranny, but there have been no repairs nor nothing to the transmission according to my knowledge (I have the 4 speed hydromatic transmission). What could be causing the incorrect speed?

 

With regards,

[rocketraider]

Several things come to mind.

 

1) tire size. If it's larger or smaller diameter and height than original tire size it will throw speedometer calibration off.

 

2) there are different speedometer driven gears available for different tire sizes. I'll attach pages from a Jan 1965 factory parts book that shows applications and part numbers. They might not be easy to find 60 years down the road.

 

3) it's 71 years old and sat frozen for who knows how long. Speedometer magnets are probably weak. Only way to fix this is remove it and have it calibrated and remagnetized.

 

From left: Tire size, axle ratio, speedometer gear ratio and part #

 

Applications:

[P6tu]

Hi,

 

Thanks for the reply. The tires should be quite exact to original size from diameter perspective. I do have fitted a bit wider tires, but I've kept the diameter close to the original.

 

So it is true that there are different gears, but am I correct that this gear is located inside the transmission and isn't exactly an easy thing to swap out or to replace?

 

If the magnets are weak, then shouldn't it be the opposite that the speedometer readings would be lower compared to the actual speed?

 

With regards,

[rocketraider]

Not 100% sure on a Slant Pan, and my early-50s manuals aren't handy, but I would think remove/replace the driven gear can be done with transmission in the car. I'll guess it has some type of retainer that can be accessed after disconnecting the speedometer cable like later transmissions, then pull the driven gear out of the tail housing.

 

You'd think off-kilter magnets would cause low speed reading but the magnets do strange things. My Toronado's speedometer was behaving same as yours and an old-time speedometer guy remagnetized it till it was within 1mph +- from 30mph to 90mph. Greatest thing was he allowed me to watch him work and explained it.