I have this gearbox that was mounted on a gage cluster I bought on ebay a while back. It has 2 speeds with a neutral in the middle, and I'm thinking that it's for adjusting speedometer speed for oversized tires, but I'm not sure. Also not sure how exactly it switches between gears and stays in a specific gear. Any help and info on this device much appreciated!
Something like that was used when a truck had a 2 speed rear end. The GMC Master Parts Book has a bunch of them. Any part number on the unit?
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
That's a speedometer control adaptor, it's vacuum actuated. The 2-speed rear ratio (HI or LO) is selected with the button on the gear shift lever which operates a vacuum valve mounted below the floor. This valve has a small port that goes to the adaptor to keep the speedometer calibrated when changing the rear axle from HI to LO.
If it came off of a TF truck it will more than likely have a ratio of 1.36:1.
Looks like it was made by Stewart-Warner and that p/n would be theirs.
Mike B
Mike Boteler
1956 Chevy 3100 Resto Rod 1952 Willys M38 Army Jeep 1983 Jeep CJ7 Laredo 1984 Jeep CJ7 +++++ Silver Spring, MD
There are seven "Two speed speedometer adapters" listed in the 1939-53 GMC Master Parts Book. All are stamped "675-" with various suffixes, but none are "DE".
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
I have a 59 gmc with the same thing attached to the speedometer Now I know what it is. The hose was missing and all the rubber components are dried up. Probably not going to put this truck on the road but if I do, I’ll have to find one of those and a new speedometer
I have one of these on my '50 Chevy COE and another on my '47 Diamond T, model 509. The latter is like yours and is vacuum actuated. The Chevy is actuated mechanically by the 2 speed selector switch. It is the same unit minus the diaphragm (and can be converted by removing it). The diaphragm tends to crack and then will fail to function due to vacuum leakage, so if yours is intact it is a valuable piece.
