Is there a rule of thumb for rear end ratios when switching from a manual to an automatic. If chevy used a 3.90 with a manual with a 283 in say a 57 1/2 ton, what ratio would be appropriate when switching to a t-350.
yes, but most all cars and trucks used a lower number on auto trans. that 57 would have been a 3.38 with a p/g 2sp I would go maybe 3.07 2.76 is pushing it for a stock 283,,,it would be doggy in city takeoffs.
Autos typically use a higher rear ratio because of the tourqe multiplication of the torqe converter. I still want to use my truck as a truck so I want to get this right.
Just for reference, the standard ratio that a '57 Chevy p***enger car had with a 283 and a 3-speed manual was 3.55, Powerglide cars came with a 3.36 gear. Not sure what rear gears the trucks used. Probably has something to do with torque converter slippage, both gears would keep the RPMs about the same on the highway with the different transmission types. Also keep in mind that a modern auto trans that uses a torque converter can pull a taller gear due to the approximate 2:1 torque muliplication provided by the torque converter. Something like a 3.55 or 3.70 would be a good gear to use when switching to the TH350.
How you drive should enter into it...if you plan on lots of highway driving over 65 mph then you probably want lower than 3.5 If you'll just drive around in town, then like he said, 3.5 to 3.7 would be fine.
1957 Chevrolet 3100 pickups ( 1/2 tons ) only came with two ratios. A 3.90 was standard and the ones equipped with OverDrive had 4.10 With the tall 15 inch tire that is usually on trucks of that era ... I would go with a later model Chevy/Gmc pickup rear end with a 3.42 ratio and the 6 lugs wheel bolt pattern. I installed a 58 Ford F-100 rear end under my 55 Chevy Cameo. I like the Ford 9 inch for ease of changing ratios. I ended up with a 3.50 ratio chunk ... but tried a 3.00, 3.25 and a 3.70 I had a SBC and a 400 Turbo ( same 3rd gear [ in Drive ] as your Turbo 350.
I have a 3.54 in one of my trucks, it's a bit much on the highway at higher speeds....not as bad as the one with the 3.90...not as good as the one that had 3.07....but how big the engine is plays into it, the 3.90 truck has a six and just isn't meant to go real fast, the others with big blocks don't need so much gear. 3.4-3.5 is a good compromise for a little V8.
You can "dial in" your rear end ratio with taller or shorter tires, more often I've gone with taller tires to tame down a 4:11 or 3:90. Although it does throw off the speedo reading. I used to "calibrate" my tach with the correct speedo readings to keep me out of trouble. I'd run next to someone and he'd beep his horn at 30, 40, 50, 60 and so on. I'd mark the tach with a sharpie and later put on stick on labels. Bob
that sounds easy...and when I think about all the time I've wasted over the years swapping out those little gears in the transmissions....
I have a '55 non-overdrive truck with a 265, 3-speed, and H033 differential with 4.10 gears. Pulled an H035 out of a '58 six cylinder non-o/d truck with 4.10s in it also. Adapting an '86 Grand Wagoneer ****er 44 with 3.42s to keep the six-bolt pattern and gain the ability to drive on freeways with the 1:1 Third gear. Added bonus: most Wagoneers had either lockers or Twin-Grip diffs. Drawback? Try finding a Wagoneer in a junkyard, or one being parted out.
another way to get decent gears is to find a rear from a 60s truck, which is the right width but probably 3.73 ratio, then get the gears and carrier from a 70s truck/van, which is more likely to be 3.07 or 3.42 ratio
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