, I'd go with a 2.73.
What diff gear ratio?
Safety Car
Joined: Jan 2008
Posts: 3,795
Likes: 435
From: Aarhus, Denmark
One of the forum members installed 4.30 rear gearing with his 6 speed because of the .5 OD to make 6th gear usable on the freeways. He actually considered a 4.56 rear end. I have had 4.11 rear end in my 79 Vette since the late 1980's because of OD trannies. It really makes a car fast and fun to drive when you close up the gearing.
I have a 11.5 pound flywheel with a 2.89 first gear in a tko600 and with my 650 hp 427 was really nice even for full powered take offs or just minimal clutch slipping just above idle.
I have a 11.5 pound flywheel with a 2.89 first gear in a tko600 and with my 650 hp 427 was really nice even for full powered take offs or just minimal clutch slipping just above idle.
I have 0.63 in 5th gear, 23lbs SFI fly wheel, and have the factory 3.70 rear end.
I'm not going to change my 3.70 to 4.11 BUT if I had less then 3.70 then I would have changed to 4.11, when I installed my 5 speed transmission !
Tech Contributor
Joined: Aug 1999
Posts: 15,173
Likes: 3,982
From: Connecticut, USA
I was not aware that 3.90 gears are no longer available. If I had to choose between the 3.70 and 4.11's I would probably go with the 3.70's. I have had cars where first gear was too deep, and cars where first was too tall. I would prefer to drive too tall. If first gear is too deep, its almost useless. You are shifting before you get halfway across the intersection. With that said, 4.11's may not be too tall for your application. I have an 18 lb. flywheel behind my 427 small block, and even with that torque you have to be careful not to bog the motor off the line.
Guess this probably doesn't help you. Sorry.
Guess this probably doesn't help you. Sorry.
Thread Starter
Burning Brakes
Joined: Sep 2017
Posts: 1,219
Likes: 365
From: Tolland CT
Mike,
I would say 95% of the diff's I built for an OD trans were 373's, some 411, few 355. You are wise to match the gear to your eng/trans.
Some notes
Toms has gears made for a 3 series case in ratios
308,336.355,373, 411, 433,456 These are USA made by US Gear. The 370 ratio is not offered as the 373 is stronger. A real 370 gear can be purchased but it will be from another company and imported. 273's are long gone. Richmond had 390s same as the GM performance parts catalog from 40 years ago. I stick with US Gear for good reason.
In addition to ratio you need to address other areas within the diff at your power level. If you are coming to the June 15th seminar I can show you everything you would want to know about these diffs.
I would say 95% of the diff's I built for an OD trans were 373's, some 411, few 355. You are wise to match the gear to your eng/trans.
Some notes
Toms has gears made for a 3 series case in ratios
308,336.355,373, 411, 433,456 These are USA made by US Gear. The 370 ratio is not offered as the 373 is stronger. A real 370 gear can be purchased but it will be from another company and imported. 273's are long gone. Richmond had 390s same as the GM performance parts catalog from 40 years ago. I stick with US Gear for good reason.
In addition to ratio you need to address other areas within the diff at your power level. If you are coming to the June 15th seminar I can show you everything you would want to know about these diffs.
if you are going to the track, yes. for pulling out at traffic lights, 11 will have you hitting second in 1 car length. 9.0 is best driver ratio. 6 speeds are really overkill. they are more for LS engines with 50 year newer computerized engine management to make the engine happy at 1900 rpm's under load.
I was not aware that 3.90 gears are no longer available. If I had to choose between the 3.70 and 4.11's I would probably go with the 3.70's. I have had cars where first gear was too deep, and cars where first was too tall. I would prefer to drive too tall. If first gear is too deep, its almost useless. You are shifting before you get halfway across the intersection. With that said, 4.11's may not be too tall for your application. I have an 18 lb. flywheel behind my 427 small block, and even with that torque you have to be careful not to bog the motor off the line.
Guess this probably doesn't help you. Sorry.
Guess this probably doesn't help you. Sorry.
After some digging, it seems that Richmond is the only major company that still offers 3.90s.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
It's actually kind of pleasant to take off with minimal clutch slipping bio
Drifting
Joined: Aug 2015
Posts: 1,939
Likes: 477
From: NSW, Australia
C3 of Year Finalist (track prepared) 2019
Try it on a simulator first and then you will start to see lower gears at the track are usually faster than higher gears.
eg. My local track I made on a simulator and tested different rear ratio's. The 3.08 was usually the fastest with the T56 magnum ratio's.
So for track days what I look for is reaching the max torque / hp RPM near the end of the straights.
eg. my zz383 rpm ~ 5500 in third gear = 109 MPH 3.08 with 3.73 rear = 90 MPH
so on a straight, if the car needs a extra shift to 4th because I reach a red line and lap times are slower. To goal is to keep the engine RPM in the max torque range and long as possible. High gears shorten the time in the max torque RPM ranges. Unless you have a very flat torque / hp curve which then again shifting costs lap times.
eg. My local track I made on a simulator and tested different rear ratio's. The 3.08 was usually the fastest with the T56 magnum ratio's.
So for track days what I look for is reaching the max torque / hp RPM near the end of the straights.
eg. my zz383 rpm ~ 5500 in third gear = 109 MPH 3.08 with 3.73 rear = 90 MPH
so on a straight, if the car needs a extra shift to 4th because I reach a red line and lap times are slower. To goal is to keep the engine RPM in the max torque range and long as possible. High gears shorten the time in the max torque RPM ranges. Unless you have a very flat torque / hp curve which then again shifting costs lap times.
Last edited by Metalhead140; May 8, 2019 at 07:49 PM.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
A stock TPI wont get 25mpg with a 3.07 and 6 spd, how on earth is anyone supposed to believe a 400hp anything with a carb is doing the same? Its NOT
OP some may get tired of me posting, but gear it for the cam not the cruise rpm. 6th isnt for acceleration anyways.
Had someone talk me into that almost 30 yrs ago....built 350, 3.08 gear. It was a pooch plain and simple. Liveable rpm on the fwy was no consolation.
OP some may get tired of me posting, but gear it for the cam not the cruise rpm. 6th isnt for acceleration anyways.
Had someone talk me into that almost 30 yrs ago....built 350, 3.08 gear. It was a pooch plain and simple. Liveable rpm on the fwy was no consolation.
Last edited by cv67; May 9, 2019 at 04:10 PM.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Good gas mileage in a C3? Wtf? 10 mpg is ok with me and my 450+ hp, 4.11 and M21. Rarely get out on the highway anyway but when I do, 3500-4000 rpm is not a problem. That's how we ran back in the day.
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Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
let me give you another opinion.. a 4.11 gear with a 2.66 1st gear is a granny gear.. you won't even use it. you will be shifting out of 1st at 5 MPH and you will find yourself starting out in second during normal street use.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
I always use 1st starting off and so what if I shift a bit sooner than when I had a 3.55. It's not a big deal like you and some others make it out to be.
Instructor
Joined: Apr 2007
Posts: 142
Likes: 6
From: Indianapolis, Indiana
This is not a Corvette, but a Chevelle. I'm running a T-56Magnum with a 2.66 1st gear coupled with 4.56 rear gears and 30 inch tires, it works very well with my engine combo (see sig below). It launches hard and I have plenty of power in the upper rpms (7k rpm) to utilize the lower gears. The 5th and 6th gear ratios are .80 and .63, which translates to a 70 mph cruise rpm at a little over 2200 rpm. The best of both worlds. 
Also, As far as 1st gear being too low with a 2.66 first and 4.56 rear gear, IMO, no its not. The lower 1st gear makes it much easier to drive on the street. Easier to get things moving, especially if you have a decent sized cam, larger heads, big intake and carb . First gear is not too short in normal street driving. With tall rear tires it takes a little effective gear out.
Also, As far as 1st gear being too low with a 2.66 first and 4.56 rear gear, IMO, no its not. The lower 1st gear makes it much easier to drive on the street. Easier to get things moving, especially if you have a decent sized cam, larger heads, big intake and carb . First gear is not too short in normal street driving. With tall rear tires it takes a little effective gear out.
Melting Slicks
Joined: Jun 2011
Posts: 3,116
Likes: 598
But with my testing when I first installed the T56 with my ZZ383 I got 27.8 MPG (23.4 MPH US) on the one test I did. Total distance driven was ~ 320km (200miles) It would have been better (more accurate) to empty the tank and use more fuel.
Driving speed was 100-115 kph ~ 62 - 72 mph. Did everything to save fuel , coasting down hills, light a steady throttle.
Then at the track a full day session ~ 250km and 82 liters of fuel. ~ 7.4 MPG (US)
Last edited by cagotzmann; May 9, 2019 at 11:11 PM.
Melting Slicks
Joined: Jun 2011
Posts: 3,116
Likes: 598
I agree with you, and at the circuit I have mostly run at I think the gearing is pretty perfect, I can run basically the whole track in third, changing to 4th on the main straight. But most of what I'm doing at the moment is hillclimbs, many of the hills are very steep, and getting off the line is an important factor too. With the steepness and high speed at many of the tracks I am slipping the clutch hard off the line and spending a lot of time with the accelerator on the floor - I suspect that a higher ratio diff gear would help with getting off the line on a steep slope and the extra torque to the tyre would probably make the car quicker than the time lost in gear shifts. I have a complete 3.7 centre that I have picked up, and all the parts to rebuild it, so I will build it up and swap it out to see if I prefer it. I am also looking at running taller wheels and tires, both for street use (for the extra ground clearance without adjusting the height) and track use (for the extra tyre footprint) which will negate some of the change in gear, so that's a factor too. I've done some spreadsheets to track all the variables, and I'm going to try it to see if I like it.
Last edited by cagotzmann; May 9, 2019 at 11:10 PM.
Drifting
Joined: Aug 2015
Posts: 1,939
Likes: 477
From: NSW, Australia
C3 of Year Finalist (track prepared) 2019
Wakefield Park is the name of the track. I'm doing well over 200km/h (120mph) before hitting the brakes on the main straight. My 3rd gear is roughly the same range as yours. Fun track!
Last edited by Metalhead140; May 9, 2019 at 11:27 PM.
Race Director
Joined: Nov 2006
Posts: 19,445
Likes: 353
From: Athens GA
St. Jude Donor '12-'13, '15- '16-'17-'18-'19, '21
4.11s in both my builds using the F-body T56 with the 2.66 1st gear.
If I want to worry about mpg, I drive my Prius.........lol
In my 67 Camaro with the first aftermarket Super T10s (nodular case instead of the aluminum one) back in early 70's I changed my stock 3.73 to a 4.10.
2.64 1st gear
World of distance when launching.
If I want to worry about mpg, I drive my Prius.........lol
In my 67 Camaro with the first aftermarket Super T10s (nodular case instead of the aluminum one) back in early 70's I changed my stock 3.73 to a 4.10.
2.64 1st gear
World of distance when launching.