I have a SM420 transmission coupled to a 3.90 differential (open driveline). The truck is a 49 1/2 ton GMC. Story is to long, and actually might not have all facts as how does this combination got made. Anyway, the transmission has the driven gear in it (perfect condition) with 12 teeth. The drive gear is missing. According to parts book it should have 4 teeth. Looked everywhere and haven't fount this gear, it has to be metal not plastic. Haven't made my mind yet over tire size, but might install 15". In Oldcarmanualproject.com states that only '54 Chevys had 3.9 gear ratio and gives the combination of driven/drive 19/6 teeth. On the GMC parts manual for a 3 speed transmission and a 6 teeth drive gear it asks for a 18 teeth driven. The ratio matches for 3/4 ton pickup prior to 1948. In summary, I don't know what the heck I'm talking about. The actual problem is I don't find metal drive gear. Also, will it affect having a a 4 speed instead of a 3 speed gearbox in the speedo reading, even more, since I have the 3.90 differential. Shall I use whatever I found and put an adapter that corrects the problem? I'm really puzzled. Thanks for any light sheded on the subject.
