Rear Axle: My axle ratio?

[50starlite]

[Tom Bredehoft]

One possibility is that your tires aren't the same diameter as original.

[Dick Steinkamp]

Dick,

How are you doing?

You are correct. 41/9 means 41 teeth on the ring gear and 9 teeth on the pinion. The gear ratio is calculated by dividing the number of teeth on the ring by the number of teeth on the pinion. 41 divided by 9 is 4.56.

By 10% slow, do you mean that when the GPS says you are doing 50 MPH the speedometer is indicating 45? That is probably within the margin of error for a car of that time period...especially if it has larger diameter tires on it now than when it was new as Tom suggests.

Another reason could be that the transmission (including the speedometer drive gear) was changed at some point in the car's history.

If the tires are the same diameter as when the car was new, and if the transmission or speedometer drive gear has never been changed, and if the speedometer was 100% accurate when the car was new, then the car had a higher gear ratio (lower numerically) when new.

[RadioRoy]

4.56 is the default/standard axle ratio for a 50 Champion with overdrive. 4.10 is optional with overdrive and standard with 3 speed and NO overdrive.

The speedometer DRIVE gear in the transmission is the same for both ratios and the DRIVEN gear on the end of the speedometer cable is different between the two ratios.

It's possible that the magnet in your speedometer has lost some strength and making the speedo read slower than the car is actually going. It's also possible that the driven gear on the end of the speedometer cable has been changed.

It's most likely that your tires are a different diameter than the stock 6-40 X 15 that came on the car.

Coker Tire's website used to list the diameters of all their tires.You could look that up and compare it to the diameter of the tires on your car.

[tbirdtbird]

in that era +/- 10% was the norm for all
somewhere in the 60s or 70s a lawsuit against GM about this changed everything, since the warranty would be up sooner than it should have been