Topic: Rear gears
in Forum: C3 Driveline Components
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Lifetime MemberOceanside, CA - USA
Joined: 11/19/2003
Posts: 191
Vette(s): 1978 Silver Anniversary model. Original L82 engine, 700r4 trans, edelbrock intake, thorly headers, rack&pinion steering, mono rear and front spring, MSD 6A box
Can someone educate me on rear gear ratio. What is the difference between a 3:55 vs 4:11 etc.
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Duncanville, TX - USA
Joined: 11/8/2003
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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
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#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Of the two you listed, the 3:55 would be considered a "high" ratio, and the 4:11 would be a low ratio.
The higher the ratio, the better the fuel mileage, and the lower the rpm of the engine while cruizin. The 3:55 was common for automatic equipped Corvettes, until around '77-'78. Then it went to around 3:36/ 3:08.
The 4:11 ratio is best for drag racing, as it gives the engine a little more help in the torque department. It is not well suited for cruizin, tho, as the engine rpms get quite high at 60/70mph...
Does this help?Adams' Apple 2007-09-20 10:27:52
The higher the ratio, the better the fuel mileage, and the lower the rpm of the engine while cruizin. The 3:55 was common for automatic equipped Corvettes, until around '77-'78. Then it went to around 3:36/ 3:08.
The 4:11 ratio is best for drag racing, as it gives the engine a little more help in the torque department. It is not well suited for cruizin, tho, as the engine rpms get quite high at 60/70mph...
Does this help?
Joel Adams
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Lifetime MemberOceanside, CA - USA
Joined: 11/19/2003
Posts: 191
Vette(s): 1978 Silver Anniversary model. Original L82 engine, 700r4 trans, edelbrock intake, thorly headers, rack&pinion steering, mono rear and front spring, MSD 6A box
Thanks Joel. That's what I needed to know.
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Bladenboro, NC - USA
Joined: 1/22/2006
Posts: 1058
Vette(s): 1972 conv., sunfire yellow, blk interior, blk vinyl and convertible top. Off frame restoration with American Speed 383 stroker engine producing 500hp plus a 200hp NOS system. MSD ignition. Hooker super competition headers with chambered exhaust.
[QUOTE=Bry58501]So what should the gear ratio be for a 4sp manual? What was it from the factory? [/QUOTE]
The standard ratio for 1972 was 3:36 but optional ratios were available.
The standard ratio for 1972 was 3:36 but optional ratios were available.
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Moses Lake, WA - USA
Joined: 4/16/2005
Posts: 2134
Vette(s): 1976 L48 4-spd, Mahogany Metalic exterior, Buckskin interior 350ci/350hp, 3rd owner, fiberglass spring, 255/50-16's Torq-Thrust II
Since we are talking diff's I have a question to ask. Most of the driving I do is interstate with a 70mph speed limit. I am thinking about changing the ratio instead of going to a 5 spd do to cost. Currently I am running stock 3.55 rear end gears with 255/50 x 16 tires which are a little smaller in diameter. Motor will be 350HO with about 350hp. Should I just stay with the stock gearing and spin the motor at 3200 to 3400 rpm on the freeway or change to something else like 3.25 or 3.08's. More interested in a crusier than a racer. Oh trans is the stock wide ration 4 speed (3 rings cut on the input shaft).
Thanks
If its any help, my 81 automatic I just put in 3:55s. BIG off the line difference then the stock gears but around 80 or so it starts to get a little winded (But theres still plenty of mph to be had).. 90% of my driving is under 70 and I love the improvement
Lifetime MemberSHELBYVILLE, TN - USA
Joined: 7/5/2002
Posts: 3942
Vette(s): 1976 L48 auto
1978 L82 4speed
1994 LT1 6speed
1978 L82 4speed
1994 LT1 6speed
Well... doing a little computing... you can go here, http://www.idavette.net/tech/ratioc.htm
just plug in the values , and click on the corresponding button at the top to figure out what you want... just going over it quickly, if you are turning around 3300 rpm with a 3.55 rear gear, you're tire diameter is 25.3 inches... it may be slightly different as it is hard to tell just what rpm exactly you are doing at 70.. but its in the 'ball park'... going to a 3.08 rear gear would drop your motor to 2863 rpm. 4th gear in a 4 speed tranny(manual) is almost always 1:1 ratio......
StingrayJim 
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Two other ways to understand rear gear ratio:
1. divide the number of teeth in the ring gear by the number of teeth in the pinion to get the ratio. Example 45/11=4.09.
2. It is also the number of turns that the drive shaft must make for one complete turn of the read wheels.
That is probably more than you wanted to know, but if it helps you understand where the number comes from, my mission is accomplished.
BTW, I will bet big $ that the gear teeth on the ring gear, pinion and diff gears were cut on a machine built by the company that has employed me for the last 43 years.
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