1963 Speedo Drive Gear Question

[Jerry Coia]

I have an 18 tooth drive gear in my 3:08 Powerglide. I am about 10 MPH short in the speedo reading. This means the car registers 40 MPH when I'm doing 50 MPH. I was told that an alternative to replacing the gear inside the PG, there is a reduction attachment that was used in the early 70's that could correct this. Can anyone help with a fix to this without replacing the gear inside the PG.

Thanks,

Jerry

[Joe L.]

Re: 1963 Speedo Drive Gear Question

Jerry-----

You could use a speedometer adapter to correct the speedo reading, but I don't think that you need to.

Powerglides used the same DRIVE and DRIVEN gears as manual trans applications. Most, if not all, Powerglides installed during the 63-67 period used the "large" DRIVE gear/ "small DRIVEN" gear combination. This is further confirmed by the fact that you have an 18 tooth DRIVEN gear installed now. If you had the "small" DRIVE gear, then this gear would not work, at all.

So, to get your speedometer into acceptable calibration, you should have the correct DRIVE gear already installed. That means that all you have to do is to install a blue, 20 tooth gear of GM #3987920. That gear should be completely compatible with your already-installed DRIVE gear.

All of this assumes that your tire diameter is close to stock. In any event, the swap to the blue gear should produce about an 8-10 mph correction, which is just about what you say that you need.