1969 speed-o gear help

[Thad L.]

My hot rod 69 is getting a new rear gear. I am going from 4.11 to 3.36. The numbers on the 4-speed trans are 3885010, there is no drain plug and the other number is P7R13. The driven gear is an orange 25 tooth gear that I believe to be GM 3860348. I don't know what drive gear is inside the trans. Can someone give me the correct the drive and driven gears so that the speed-o will read correct? Thanks Thad

[Joe L.]

Re: 1969 speed-o gear help

Thad-----

First off, the transmission is not correct for a 69. The 3885010 main case casting number and production code would indicate a 1967 transmission. Thta's not really a functional problem for you; this transmission is configured very closesly to the 69 transmission that was originally installed in your car. However, to solve your speedo gear change it is VERY important to understand that you have a 67 transmission and not a 69 transmission. Also, my gear recommendations assume that you have stock or near-stock diameter tires.

In order to change your speedometer gear configuration to accomodate the swap from 4.11:1 gears to 3.36:1 gears you are going to have to change BOTH the speedometer DRIVE and DRIVEN gears. The orange 25 tooth gear is the correct gear for your 4.11:1 ratio and it works with a GM #3708145 DRIVE gear. This DRIVE gear is a steel gear. For a 3.36 rear gear ratio you will need to change the DRIVE gear to a GM #3708144 and the DRIVEN gear to a GM #3987920 BLUE gear. The 3708144 steel DRIVE gear is GM-discontinued but may be available used, aftermarket, or from a 63-67 specialty vendor like LICS. The 3987920 gear is still available from GM. To change the drive gear, you will have to remove and partially diassemble the transmission. The only other option is to use a speedometer adapter. For this option, it would be best to take the car to a speedometer shop for fitment and calibration.

As a side note, for 1968-70 Muncies, the speedometer DRIVE gear was a nylon gear which was retained to the main shaft by a special clip. The 68-70 DRIVE gear cannot be used on your 67 mainshaft (unless it's been changed to a 68-70 mainshaft which was, for awhile, the SERVICE mainshaft for all 63-67 Muncies). The 63-67 DRIVE gear can be used on a 68-70, but not vice-versa. That's why I said it was important to know that your transmission was a 67 model and not a 69. The transmission DRIVE gear difference is one of the important differences between the 63-67 Muncies and the 68-70 Muncies.

[Duke W.]

Re: 1969 speed-o gear help

If you don't want to disassemble your gearbox to replace the drive gear you could bring the speedo into accuracy by having an adapter installed with the correct gearing to bring the speedo into accuracy. The easiest way to do this it to reinstall the 25T driven gear and take the car to a speedometer shop to have the speedo calibration tested and the properly geared Stewart Warner adapter installed.

The proper adapter will also correct for non-OEM revs/mile tires.

Duke