My 59 SM420 was originally hooked to a NAPCO 4wd unit on a 3/4 ton. I am using it with my 59 1/2 ton with stock 235. It has a stock 1958 4:11 rear axle.
The 59 SM420 has been totally rebuilt and is really sweet for country rides and even the once and a while 50-55 mph highway ride but I have tried 3 different speedo drive gears in the side of the gearbox; a 11,12 and 13 tooth and none give me right speeds and NONE go over 35mph.
I noticed the teeth are different shape on the 11 and 12 than the 13. The finer teeth on the 13 are closer to correct than the squared teeth on the others. Were the speedo drive gears different for different years? How many ratios could they have had? The speedo is new and working fine......Tried 2 different cables also and they do not change the accuracy.... Is there a drive with more than 13 teeth? Thanks
