Rear Differential Ratios
Thread Starter
Staging Lane
Joined: Oct 2011
Posts: 67
Likes: 2
From: Springfield, OH
Rear Differential Ratios I am doing a frame off rebuild of a 1971 Lemans convertible, it will have LS1 power but I am not sure about the transmission yet (will be T56 or 4L60e).
I am currently in the process of tearing down the 12 bolt rear differential to rebuild it and discovered it has a 2.73 ratio gear. My first thought was wow, that's not going to perform very well. Then I did some research into the cars the LS1 comes from, the Camaro and Corvette, and discovered they both used 2.73 rears with the automatic transmission models. Manual transmission cars had 3.42.
So I am thinking if I do go with the automatic, if this ratio was good enough for the Corvette it should be good enough for me? Or am I missing something (I do realize the Lemans may be a little heavier and that will affect things somewhat)? I will probably run 18 inch wheels. I will not be racing this car, I am just looking for all-around good driveability and performance on the street.
I would rather not have to spend the money to upgrade the rear, that will probably run several hundred dollars for a new carrier and gear set.
Thanks,
Phil
I am currently in the process of tearing down the 12 bolt rear differential to rebuild it and discovered it has a 2.73 ratio gear. My first thought was wow, that's not going to perform very well. Then I did some research into the cars the LS1 comes from, the Camaro and Corvette, and discovered they both used 2.73 rears with the automatic transmission models. Manual transmission cars had 3.42.
So I am thinking if I do go with the automatic, if this ratio was good enough for the Corvette it should be good enough for me? Or am I missing something (I do realize the Lemans may be a little heavier and that will affect things somewhat)? I will probably run 18 inch wheels. I will not be racing this car, I am just looking for all-around good driveability and performance on the street.
I would rather not have to spend the money to upgrade the rear, that will probably run several hundred dollars for a new carrier and gear set.
Thanks,
Phil
Last edited by fsdproject; Oct 28, 2011 at 04:39 PM. Reason: Added driving info
Gear ratios in the 4L60e are: 1st 3.059, 2nd 1.625, 3rd 1.000, 4th .696
Gear ratios in the T56 are: 1st 2.66, 2nd 1.78, 3rd 1.30, 4th 1.00, 5th 0.74, 6th 0.50
4L60e has taller first gear, thus the reason for the 2.73s. I'd keep the 2.73s with the '60e unless you decide to rebuild the rear in the future. Now, say, if you had a 4L80e (2.48/1.48/1.00/.75), I'd put a taller gear in (such as 3.73s).
Gear ratios in the T56 are: 1st 2.66, 2nd 1.78, 3rd 1.30, 4th 1.00, 5th 0.74, 6th 0.50
4L60e has taller first gear, thus the reason for the 2.73s. I'd keep the 2.73s with the '60e unless you decide to rebuild the rear in the future. Now, say, if you had a 4L80e (2.48/1.48/1.00/.75), I'd put a taller gear in (such as 3.73s).
I think that the answer depends upon what you'll expect/how you'll be driving the car.
For example, I found that the LS1 doesn't really 'come on' until about 3200 rpms. I'm used to old school, where power is produced at around 2400.
After alot of reading here, I decided to go with a 3.70 gear. It works really well from a dig, and from this experience, I'd even go a bit more, to say 3.90.
This is with a manual 5 sp. w/.063.
For example, I found that the LS1 doesn't really 'come on' until about 3200 rpms. I'm used to old school, where power is produced at around 2400.
After alot of reading here, I decided to go with a 3.70 gear. It works really well from a dig, and from this experience, I'd even go a bit more, to say 3.90.
This is with a manual 5 sp. w/.063.
TECH Resident
Joined: Sep 2009
Posts: 859
Likes: 2
From: Frozen Tundra, WI
There was a posting on here a while back about someone who did a bunch of research into rear gears, converters and ET. He said that his research came to the conclusion that...if you have a good stall of 3000+ then the rear gears made very little difference...only about 1/10 in ET and definitely not more than 2/10ths. I would stick with the 2.73's and get a high torque multiplication (2+ for the street) converter. With a lock-up you will like the MPG's better too. There is no need to run steep gears with these motors. I believe it is because of the flat torque curve of these motors that comes on early and doesn't peak until 4000+ that makes this possible. The traditional SBC needed torque multiplication help with steeper gears (numerically higher).
Staging Lane
Joined: Jul 2007
Posts: 52
Likes: 0
From: Chicago, Sw Subs
Gear I have a 67 nova with LS1 / T56 and i just upgraded my RR and went with 4.10 gears, Auburn, Axles i am running a 245/45/17 drag radial and it runs about 2k at 65-70.
The car pulls hard i am very glad i did the 4.10 instead of 3.73 i have a 10 bolt 8.2 so i had limited option on gears.
Steve
The car pulls hard i am very glad i did the 4.10 instead of 3.73 i have a 10 bolt 8.2 so i had limited option on gears.
Steve
Staging Lane
Joined: Apr 2009
Posts: 76
Likes: 1
From: Ohio, USA
I have several GM vehicles that were stock with 3.42s & 4L60E. All get great mileage, run about 2k RPM @ 60mph and perform great. Also had a 5spd 5.0L Mustang with a close ratio 5spd and 3.55s...ran great, great on the highway and fantastic MPG.
I would also consider 3.42s with a T56 if you get one with a lower 6th gear. IIRC some have a 0.70:1 6th, which would work well with 3.42s. Otherwise, I'd run at least 3.73s to keep the cruise RPMs up.
I would also consider 3.42s with a T56 if you get one with a lower 6th gear. IIRC some have a 0.70:1 6th, which would work well with 3.42s. Otherwise, I'd run at least 3.73s to keep the cruise RPMs up.
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I am doing a frame off rebuild of a 1971 Lemans convertible, it will have LS1 power but I am not sure about the transmission yet (will be T56 or 4L60e).
I am currently in the process of tearing down the 12 bolt rear differential to rebuild it and discovered it has a 2.73 ratio gear. My first thought was wow, that's not going to perform very well. Then I did some research into the cars the LS1 comes from, the Camaro and Corvette, and discovered they both used 2.73 rears with the automatic transmission models. Manual transmission cars had 3.42.
So I am thinking if I do go with the automatic, if this ratio was good enough for the Corvette it should be good enough for me? Or am I missing something (I do realize the Lemans may be a little heavier and that will affect things somewhat)? I will probably run 18 inch wheels. I will not be racing this car, I am just looking for all-around good driveability and performance on the street.
I would rather not have to spend the money to upgrade the rear, that will probably run several hundred dollars for a new carrier and gear set.
Thanks,
Phil
I am currently in the process of tearing down the 12 bolt rear differential to rebuild it and discovered it has a 2.73 ratio gear. My first thought was wow, that's not going to perform very well. Then I did some research into the cars the LS1 comes from, the Camaro and Corvette, and discovered they both used 2.73 rears with the automatic transmission models. Manual transmission cars had 3.42.
So I am thinking if I do go with the automatic, if this ratio was good enough for the Corvette it should be good enough for me? Or am I missing something (I do realize the Lemans may be a little heavier and that will affect things somewhat)? I will probably run 18 inch wheels. I will not be racing this car, I am just looking for all-around good driveability and performance on the street.
I would rather not have to spend the money to upgrade the rear, that will probably run several hundred dollars for a new carrier and gear set.
Thanks,
Phil
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