Weird Circa 1955 GMC 7-speed truck Hydramatic??? Has anybody here ever heard of, seen or operated a GM 7-speed Hydro? Apparently they were used in some GMC heavy trucks starting in 1955 and consisted of two, more or less conventional GM 4-speed Hydramatics, *supposedly* stacked one on top of the other (not sure, but perhaps in a common case???) and coupled up so the engine power traveled in series through the two coupled gearboxes giving two ranges and a total of 7 forward speeds with a shift sequence of 1-1, 1-2, 2-1, 2-2, etc., and with the top two gears being overdrive ratios. Any information and especially any pictures of one of these very oddball and apparently fairly rare and little-known transmissions would be greatly appreciated. Mart3406 ==========================[FONT=Verdana, Arial, Helvetica, sans-serif][/FONT]
Chevy medium trucks of that era used a Powermatic transmission, which was a 6 speed, MT-25 and MT-30. I think it's what we'd probably call an Allison transmission. Had a lock up converter too.
I was in high school in those days, graduated in '56. The school fronted on US 41 and the campus was 2 blocks long with a traffic light at the cross street at each end. So almost every truck that passed in either direction was pulling away from a stop when it passed the school. A freight hauler named Terminal Transport, IIRC, bought a big fleet of GMC semis equipped with automatic trans. and Jimmy 2 stroke diesels. When thay came by it was a symphony of endless gear changes and GMC Roots blowers whining. A big distraction to a car crazy kid! Dave
I never heard of a 7 speed but I know they used the hydromatics in the city transit buses which I presume were 4 speeds.
Sounds like something you'd see in the Motor's Truck Repair Manual and wonder WTF they all wound up, I remember marveling at the GMC V12 after looking in one of those books...
Now that I think about it a bit more, it wouldn't surprise me if it was 7 speeds and a 2 speed axle, because believe me, the gear changes in those 2 blocks past the school were endless! Dave
I worked on a truck with one back in the early 60s. This one had a 389 Pontiac in front of it, but it was not the original engine. It probably came with a 316. 56 GMC It also had a Highway tandem conversion. An two speed auxilary trans with two output shafts, one driveshaft to the front rear end, and a two piece shaft over the front rear into the rear rear. Anyway, it was as discribed, two 4 speed trans, one on top of the other. As I recall the shifter was only maked for high and low, instead of all speeds.
With respect to Old Sub the picture he sent in is a Fuller thirteen speed roadranger. Note the back box (bolted on) is the aux section. I do know that a lot of mid fifties GMC used a automatic with a seperate aux box made by Clark mfg(very rare). thanks for your time.
The placard on that truck described it as one of those twin hydro matics. I've never seen one other than this one, so if the placard is wrong its wrong. These photos were taken at an ATHS show so I'd have expected some people to speak up if the guy was misrepresenting the truck.
i used to drive a 1957 chevy 10 wheeler dump truck with a 322 ci buick motor, allison 6 speed automatic transmision, 3 speed browning, power divider, and retarder that worked with the torque converter to keep from running away on hills. the owner bought it new. the cab was full of shift levers! i have also seen chevy trucks of the later 50's with pontiac motors
I have seen one of those transmissions, it was in a military duece and a half with a 302 GMC engine. It was kinda fragile, I think the truck is still behind my buddy's house. Old time
The trans set-up explained on that plaque is displayed on the back of that truck. It has been replaced with a modern trans. Search the ATHS.org site for more info.
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