I have a 61 chevy truck that has a granny gear with a 235.. Wondering who has tried to up the rear end gear to make the granny gear a "useable gear".. Im considering puttin this set up in my 48 fleetmaster, and thought maybe i could run a 273 in the rear to make the gear a little longer?? Im sure someone has tried this?!?!?
What are the gear ratios in the trans? Spread between 1st and 2nd will give a good idea of how it will work.
It's likely an SM420 or SM465, which would give you a first ratio of either 7.05 or 6.55, with a second gear ratio of 3.58.
My 66 GMC has an SM420. First gear is 7.05:1. If reverse is toward the driver and back, then its probably an SM420 in your truck.
personally, i'd ditch the truck trans, and use the six with a different gearbox. pissing around with a rear gear low enough to make the "granny gear" useable is going to bring you down into the low 2.x:1 territory, and good luck finding a set
If you Just want to use the trans to have a floor shift in the car and because you had it it's just a three speed with an underdrive gear that you don't use except when it's really needed. I've had a lot of trucks with compound fourspeeds over the past 40 years and never used the granny gear in any of them unless I needed the extra low speed grunt. High gear is still 1 to 1 and that is what you need to pick your rear end gears for. I bought a 46/47 Ford truck granny box to go behind the 254 Ford flathead six in my roadster only because it was available and it was a whopping 50.00 with the hogshead on it and a throwout bearing. Unless I need to drive at parade speed the car will probably never need to be driven in Granny and it will just be along for the ride.
I didn't look up the ratios for your Trans. But I'm thinking that the jump from granny to the next gear is going to be too big. Will leave you in a flat spot on the torque curve between those two gears. I also think it will be a bear to shift out of granny into next gear because the speed of the output shaft would be increased due to the rear gear change. Could always try it though.
my 62, 3/4 has a 456 gear. finding parts for that Eaton differential is impossible. i know because i tried. i recommend a T5 trans swap. with my T5 and 16" wheels, my truck now drives nicely at highway speeds.
I swapped a 3.73 rear into my 57 chevy one ton to replace the 5.14 original rearend. Now I need to start out in granny low. But the old 283 in it is really tired.... I like the idea of going to a milder rearend gear and having a steep transmission low gear. Just beware that if you have a very torquey engine (like a big block) then you're putting a big load on the driveshaft, etc in low gear. On the plus side, the driveshaft does not have to spin fast goign down the highway, and you also have a more efficient drivetrain in high gear, compared to using overdrive. Overdrive adds gears at the trans and uses a steeper gear at the rearend, which makes it less efficient.
Truck transmissions belong in trucks! Too big a ratio jump between 3 and High for a car. Find a 3 speed. Lots of them around.
Here is another way to look at it: If you can find a late 70s 10 bolt, alot of them have a 2.43 gear ratio. I have one in my 51 Chevy with a 350/350 and, even though I thought I would hate such a tall gear, I love it. But I am not sure how a 235 would like that tall of a gear, so a granny may be nice.
just drive it like a 3 speed. like was said once above. when you need to crawl along in traffic that low 1st is really nice. when you need to back up the low reverse is perfect. I had little p-10 van that had 3 speed in it and replaced it that type trans. when i scrapped that i bought a 3 door suburban and transfered that trans to it. it's there when you need it. unless you are terribily under powered, 3 is all any car needs. frank
A few rods used them long ago, probably because they were available cheap. The idea of a tall rear end ratio and a somewhat useable first gear works fine in practice too, although there is a big space between first and second. If you have a rear end that allows easy gear changes, like a 9" Ford, start with a 2.79 or so and see how it works. You can change the gears to get what you want that way. Most pickups that used 4 speeds also had low rear ends, so they weren't very freeway friendly, and that was often blamed on the trans even though they all used the same 1:1 final ratio.
Got the same issue with our 65 chev..went to a larger tire ,but still seldom use first gear..cruises 55 mph at 2800 RPM's.. Plan to dump this rear end for a 73 and newer style, better gear selection. Rick
I actually did the originally in my '55 ranch wagon it gave me a floor shift and 3 gears that I didn't have to doube clutch to get into first. Only used granny once when I was pulling a rich kids late model out of my parking spot in the parking lot. One of my buddies finally came up with a full syncro 3 speed from gawd knows what, but it was a bolt on and a Fenton shifter. I think out of the same car.
During and before my teen age years my uncle Jim's hunting rig was a 50 or so Dodge or Plymouth tudor wagon with a Chrysler six and a Dodge 2-1/2 ton truck granny transmission. That thing would climb a rock wall in low gear but drove great on the road and he used it daily and for hunting for probably fifteen years before he bought his first 4x4 pickup.
Thanks all for the feed back.. Still un decided what to do.. Really want to run a floor shift on a budget, and have this one already.. I guess time will tell.. If i sale the truck in the near future ill go with something else.. if not ill make it work for a while!!
Look around for a 3-speed/OD manual floor shift transmission from a '81-87' Chevy 1/2T. It's a NPG transmission Chevy purchased from MOPAR and used for a while. There're two varietys: one has the MOPAR trans-bell bolt pattern and will need the GM bell. The other has the conventional Chevy bolt pattern.
I've used the trannys Tagman mentions in two old GM trucks.The OD is nice,kinda ride ratio but good for a stock engine. I just swapped in a 80 Chevy 1/2 4x4 rear axle in my 50 Chevy pu.Keep the SM420 truck tranny ,change the rear bearing retainer for one from a later open drive trans.The axle has 3.42 gears with tall 16 inch radial truck tires.A stock 235 has no problem moving the truck along even with occasional loads and starting off in second gear.
so the truck sold and im back to the drawing board.. Still want to run a stick, so im gonna try to look for a good donner car like a nova or something i can use the whole runnin gear.. again, thanks for all the feed back!!!
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