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Old 09-20-2013, 08:36 PM
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new rear end for 46 chevy couple

changing rear end on 46 chevy couple. keeping 250 6 cyclinder that is in car now. its like new. putting turbo 350 behind it and have to change rear end because of torque tube and the rear that is there is wore out. what would be the best gear ratio to put in to run at highway speeds of 75 with out over reving the engine. thanks

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Old 09-21-2013, 01:05 AM
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Originally Posted by rockytop57 View Post
changing rear end on 46 chevy couple. keeping 250 6 cyclinder that is in car now. its like new. putting turbo 350 behind it and have to change rear end because of torque tube and the rear that is there is wore out. what would be the best gear ratio to put in to run at highway speeds of 75 with out over reving the engine. thanks
What size tires are you using out back?
Assuming 27" your cruising speed of 75mph your going to want no more then a 2.73 gear.
It really depends on what rpm you want to be at. 2500rpm seems like a comfortable cruising rpm for a stockish 6.
The problem then becomes its real doggy out of the hole.
You have to find a compromise your willing to live with or step up to a 700R4 and have your cake and eat it to.
A 2.73 and a 27" tire in a heavy car is going to suck out of the gate.
A 3.55 gear will be far better.
A 3.55 with a TH350 and 27" tire is 3300+ rpm
A 3.55 with a 700R4 and 27" tire is 2300 rpm.
See what I mean?
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Old 09-21-2013, 08:22 AM
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The 3:50 or 3:55 IMHO is the best gear. With the six cylinder you do not want a gear in the 2's.

Vince
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Old 09-21-2013, 08:38 AM
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thanks for your help. trying to find one with in an inch of over all width and both that i found are 2.73 and 3.08 chevy. wanted to run the white walls that are on the car. they are 6.00 r 16. so i will keep looking. thanks for the info.
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Old 09-21-2013, 08:53 AM
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the tires that are on the car are 28.36 in diameter 16 inch
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Old 09-21-2013, 09:13 AM
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the tires that are on the car are 28.36 in diameter 16 inch
Yah, with that tall a tire your going to want to stay away from 2 series gears......as long as you either lower your 75mph cruising speed, or live with 3000+ rpm a 355 will be your best bet if u stick with the th350
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Old 09-21-2013, 09:23 AM
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thanks if i go to the 700r4 will that improve matters?
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Old 09-21-2013, 10:17 AM
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Also consider the 200 4R. This transmission has better gear ratios than the 700R4, is the same length as the TH350 and is usually easier to install than the 700. With either one of these transmissions you could use as much as a 3.73 gear and still be able to run down the highway in comfort, and get away from a stop light quicker.

That 250 engine isn't going to require a very strong rear end, so a 7.5 10 bolt would be just fine for it. Look at S10 rear ends for a easy fit. The 2 wheel drive rear ends are about 55 inches wide, 4 inches narrower than the 4 wheel drive rear ends. Also look at G body rear ends. These are 58 inches wide, which might be pretty close to what you have now. The 1st gen Camaro rear ends and '68 and later Nova rear ends are 60 inches wide, and the 2nd gen Camaro rear ends are 61 inches wide.
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Old 09-21-2013, 10:17 AM
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thanks if i go to the 700r4 will that improve matters?
In every single way yep. You can get a little more aggressive with gearing to get the car out of the hole much better, and still cruise at low rpm and get good mileage.
A set of 373 gears with the 700's over drive will let you cruise at 75mph at 2300rpm all day.
And with the short first gear and 373's it will accelerate very nicely from a stop.
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Old 09-21-2013, 10:20 AM
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BGH is right, the 200 is an excellent choice too, the only reason I went with the 700 is availability and the deeper overdrive.
But either trans will suit your needs.
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