Requesting input on rear end for SWC
08-10-2012, 05:18 PM
#1
Instructor
Thread Starter
Requesting input on rear end for SWC
My '63 FI SWC is making occasional noises in the the rear end differential. It currently does not have the stock or original one, but instead has a 3:36 for a big block 454 automatic. Therefore I am looking at replacing it with a date coded one, possibly a 3:55 or 3:70 (it is a driver). It's running about 2900-3000 rpm's at 70 mph currently. Anyone out there that's running with a 3:55 or 3:70 and can tell me what RPM's they're at 70 MPH? Any other suggestions pro or con with the 3:55 or 3:70 gear ratio?
08-11-2012, 12:07 AM
#2
Cruising
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Back in the day when the tire diameters were about 27-1/2", I always used this very simple rule of thumb.
Drive the car at 82 mph, in 4th gear (1:1 in a stock 4-speed) and note the tachometer reading. If it was 4,500 rpm you had 4.56:1 rear end; 4,100 rpm you had a 4.11:1; 3,700 rpm you had a 3.70:1, and 3,100 rpm would mean a 3.08:1.
If you want to obey the law, just cut the speed in half, and the tach readings will be reduced by half too. For example 42 mph, 2,050 rpm = 4:11:1 rear end.
JDB
Drive the car at 82 mph, in 4th gear (1:1 in a stock 4-speed) and note the tachometer reading. If it was 4,500 rpm you had 4.56:1 rear end; 4,100 rpm you had a 4.11:1; 3,700 rpm you had a 3.70:1, and 3,100 rpm would mean a 3.08:1.
If you want to obey the law, just cut the speed in half, and the tach readings will be reduced by half too. For example 42 mph, 2,050 rpm = 4:11:1 rear end.
JDB
08-12-2012, 05:17 PM
#3
Instructor
Thread Starter
Back in the day when the tire diameters were about 27-1/2", I always used this very simple rule of thumb.
Drive the car at 82 mph, in 4th gear (1:1 in a stock 4-speed) and note the tachometer reading. If it was 4,500 rpm you had 4.56:1 rear end; 4,100 rpm you had a 4.11:1; 3,700 rpm you had a 3.70:1, and 3,100 rpm would mean a 3.08:1.
If you want to obey the law, just cut the speed in half, and the tach readings will be reduced by half too. For example 42 mph, 2,050 rpm = 4:11:1 rear end.
JDB
Drive the car at 82 mph, in 4th gear (1:1 in a stock 4-speed) and note the tachometer reading. If it was 4,500 rpm you had 4.56:1 rear end; 4,100 rpm you had a 4.11:1; 3,700 rpm you had a 3.70:1, and 3,100 rpm would mean a 3.08:1.
If you want to obey the law, just cut the speed in half, and the tach readings will be reduced by half too. For example 42 mph, 2,050 rpm = 4:11:1 rear end.
JDB
Which rearend do you all prefer for a mid-year small block (Fuelie) which is a driver, and why? Thanks
Last edited by Montana Mike; 08-12-2012 at 05:19 PM. Reason: grammar
08-12-2012, 05:30 PM
#5
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I had a 1964 F.I. Corvette that I ordered new, but it came with 3.70:1 Positraction by mistake. I did not like the way that felt in comparison to my other cars, so I had the dealer order a 4.11:1 replacement.
3.70:1 would be the ratio that I would prefer for the Big Block 396 - 427 - 454. The added torque of those engines matches up with that ratio much better.
JDB
08-12-2012, 05:39 PM
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If you are showing 3,500 rpm at 70, you would be doing 4,100 rpm at 82 mph (3,500 / 70 X 82 = 4,100).
So if your tires are about 27-1/2 inches in diameter,
and your speedometer reading is correct,
and your tachometer reading is correct,
and your speedometer gear in the transmission is the correct one,
note clutch slippage is not important because that would only affect your actual miles per hour, not the readings shown on the factory installed speedometer which gives a reading based upon the input from the speedometer gear in the transmission.
I think you have a 4:11:1 rear end ratio, not a 3:70:1.
JDB
So if your tires are about 27-1/2 inches in diameter,
and your speedometer reading is correct,
and your tachometer reading is correct,
and your speedometer gear in the transmission is the correct one,
note clutch slippage is not important because that would only affect your actual miles per hour, not the readings shown on the factory installed speedometer which gives a reading based upon the input from the speedometer gear in the transmission.
I think you have a 4:11:1 rear end ratio, not a 3:70:1.
JDB
08-12-2012, 06:27 PM
#8
Melting Slicks
Depends on where, and how you drive. If you do a lot of open road driving, and you just like to cruise......then a 3:08 or a 3:36, would be a good choice. But if most your driving is in town, and you enjoy putting your foot in it every once in a while, then a 3:70, or even a 4:11 would be a good choice. I had my rear rebuilt, and I kept the 4:11. I love the "pin you against the seat acceleration", and the sound of that high revving small block. 
08-12-2012, 07:25 PM
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08-12-2012, 07:26 PM
#10
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08-30-2012, 01:44 PM
#12
Race Director
I agree with John...I ordered my 63 with a 4.11 and by using the tach for speed, I just doubled the tach reading ....ie. 3000 rpm equals 60 mph.......3500 equals 70 and so on
08-30-2012, 02:08 PM
#13
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If it came with the 250hp or 300hp engine, it had a 2.54:1 first gear; if it had the 340hp or 360hp engine, it had a 2.20:1 first gear. There was no customer choice - the wide-vs-close ratio 1st gear was dictated by the engine.
08-31-2012, 02:33 PM
#14
Racer
Mine would be the 2.20 frist gear I suppose I should put a big shafted muncie with a 2.54 first gear since I have some
I agree with Ironcross and JohnZ.