The Stovebolt Site Forums The Shop Area Engine Shop Speedometer drive gear 49 GMC 4 speed

Have 49 GMC 3/4 ton with GM591865 4 speed transmission, enclosed drive shaft and a 5.14 rear end. When I went to hook up the speedometer cable, I discovered that the previous owner had substituted a wooden plug for the drive gear into the tranny. Have found a couple of drive gears, but none that fit correctly. The closest speedo drive gear that I have found has 13 teeth. Guessing that I need 8-9. I pulled the drive shaft off the rear of the transmission so I can actually match up the sppedo gear with the spiral gear on the tranny mainshaft. Can the drive gears be swapped out and if so, how do you remove from the housing. I have not gotten a real good look in the end where the speedo cable inserts, but it looks like a small snap ring type arrangement down inside may hold it together. No speedometer shops close either, so trying to work through local parts houses. Any suggestions or contacts that could help would be appreciated. If you have the correct gear, that would be even better.

My first question would be... why did the P/O put a wood plug there?

I would call Nevada Speedometer service. They do rebuilding and calibration of speedos. You should have an early sm420 (MUNCIE) transmission in there. Most 3/4 tons came with 4.57 gears, and most 1 tons came with 5.14 gears. Does the torque tube go all the way from the transmission to the rear axle? How many lugnuts on the wheels? it sounds like you may have a 1/2 ton with 4.11 gears by your description. Verify your gears, and take a measurement of the distance traveled by one complete rolling tire revolution.

John

John
Not sure why the P/O did some things. He had even taken the steering column apart(lost a lot of parts).The old wooden plug had been there for some time. I did not even notice it when I put the tranny in originally. The plug was painted the same color as the transmission and did not stick out very far. Will look at the tranny more closely to see if any further stamping on housing to check manufacturer, etc.
I have already verified that this is a 3/4 ton 8 bolt wheel 5.14 rear end truck. The 5.14 rear end and 4 speed tranny were options on the Hypoid GMC H-050(3/4 ton) per the GMC Maintenance Manual I have. Had already verified the rear end ratio early on since I had to put it back into the rear axle housing along with axles, etc. Looked up the Serial Number and also verified that it is a 150 series made in mid 1949. The drive line has a torque tube back to the carrier bearing and open shaft from there to the rear end. I am fairly certain that all above is correct, but would not be the first time that I missed something on this project. I have had to start from scratch on this one since almost everything had been disassembled and moved a few times before I bought. The second truck that I had bought for parts was pretty incomplete also.

The speedo spins up and seems to be working ok. Not real concerned about accuracy since I plan to keep the 5.14 rear end and just cruise local streets. Doubt that I am in risk of getting too many speeding tickets with the set up that I have. If I upgrade rear end or tranny, will definitely get speedo accurate. Right now, just working toward actually pulling her out of the shop under own power. I had nothing above the frame when I started, so still have truck on dollies with last section of driveline still not installed. I will put that in in next week or so when I need to pull truck out to paint doors and fenders. Right now I have the cab, inner fenders, and some stuff hung on the radiator supports. All outer fenders, doors, hood, running boards, etc are still off truck waiting for final body work and/or paint.

Thanks
Willie