Change Rear End Gears??
Thread Starter
2nd Gear
Joined: Jul 2007
Posts: 2
Likes: 0
Change Rear End Gears??
I have 74 L82 with Close Ratio 4 speed and I am looking to change rear end gears to maybe actually have some low end performance!!! Can anyone tell me a good gear to run to gain, but still be able to run on the highway without screaming RPM's? Also do I have to totally drop the rear end to do that or can I split it and pull the housing out the front?? Any help would be much appreciated.
Premium Supporting Vendor
Joined: May 2000
Posts: 4,312
Likes: 70
From: Clearwater FL
3:55's are a great all around gear in my opinion. You will have to drop the rear to do the gear swap but you don't have to have everything out. You can undo the strut rods and half shafts and pull it out from the back. You'll have to shimmy it around the exhaust, unless you have side pipes but it can be done without pulling everthing out.
Drifting
Joined: Jul 2007
Posts: 1,712
Likes: 0
From: Tampa FL
Depends on what you have now (not sure what it came with, but unless you know the car's history there is no guarantee it's original so you may want to spin the wheel and check - a previous owner may have already upgraded it). If you're at 2:72 or 3:08 you'll like the 3:55, but if you have a 3:36 you won't see much difference and would probably want to go to 3:70 or 3:73.
Safety Car
Joined: Oct 2006
Posts: 3,930
Likes: 15
From: Calgary Alberta
Depends on what you have now (not sure what it came with, but unless you know the car's history there is no guarantee it's original so you may want to spin the wheel and check - a previous owner may have already upgraded it). If you're at 2:72 or 3:08 you'll like the 3:55, but if you have a 3:36 you won't see much difference and would probably want to go to 3:70 or 3:73.
Melting Slicks
Joined: Nov 2003
Posts: 2,362
Likes: 1
From: San Antonio TX
I was informed that 3.70's could be purchased but not cheap from TrackDogg or Mike. You might wish to check with him on that here on the forum to find out where from.
But you can also go with the 3.55 gear set.
But you can also go with the 3.55 gear set.
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,990
From: Connecticut, USA
You need to know what you have for a tranny to help you select the best gear. Ie: a CR tranny wouldn't be a good choice with a 308 or even 336 gear. A WR or auto would work with just about any.
To my knowledge the only gears avail for the 63-79 diff are made by Richmond,US Gear( Tom's are US), Yukons. The best in my opinion to use are Tom's and he is out of 336 &373 until 09. Many Vendors buy from Tom and may still have them, if so you will pay a lot for them. Some will hold them for rebuilds as the profit is higher then selling he gears outright. Some may not tell you whose gears they have it they are possibly imported.
The last time Tom's 373's were back ordered it was many many months before they arrived.
308 355 and 411 are in stock.
To my knowledge the only gears avail for the 63-79 diff are made by Richmond,US Gear( Tom's are US), Yukons. The best in my opinion to use are Tom's and he is out of 336 &373 until 09. Many Vendors buy from Tom and may still have them, if so you will pay a lot for them. Some will hold them for rebuilds as the profit is higher then selling he gears outright. Some may not tell you whose gears they have it they are possibly imported.
The last time Tom's 373's were back ordered it was many many months before they arrived.
308 355 and 411 are in stock.
Instructor
Joined: Jan 2007
Posts: 195
Likes: 2
From: Wilmington MA
I went from 2.72 to 3.54 and love it. Plenty low end and reasonable on the highway. I have a 4spd and an l48 but with a l82 style cam so were making about the same power maybe 250 or so. Zip did mine for $800 incl shipping. new gears/bearings/seals/yolks. good luck, have fun.
If you want, i can get my chart re rpm Vs mph so you can see exactly what your highway speed would be at any given rpm. That was a must for me before i picked the gear set.
If you want, i can get my chart re rpm Vs mph so you can see exactly what your highway speed would be at any given rpm. That was a must for me before i picked the gear set.
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Melting Slicks
Joined: Nov 2003
Posts: 2,362
Likes: 1
From: San Antonio TX
For me the way I came to a decision as far as rear end gears is to think of what rpm I will wish to turn the engine max and what max speed I would realistically run. So with my itsy bitsy teeny tiny BIG BLOCK of 468CI and a realistic redline of 6500 I felt that 140MPH would be fine and still give me some good fun street manners. As this will not be my primary vehicle nor something I would run from sea to sea so MPG was not a huge concern.
So to be honest you have to be honest with yourself as far as what you want the car to do. And what trans you will run and what is the capability of the engine you have or what you plan to upgrade to as far as TQ and RPM capability.
So to be honest you have to be honest with yourself as far as what you want the car to do. And what trans you will run and what is the capability of the engine you have or what you plan to upgrade to as far as TQ and RPM capability.
Race Director
Joined: Jan 2003
Posts: 15,176
Likes: 88
From: State of Confusion
MPH x GR (Gear Ratio) div/by .006 div/by 1/2 TH (Tire Height) = RPM needed to run that MPH
~example~
if you run a 27"-tall tire with 3.55:1 gears, it'll require approx 3100 RPM to run 70 MPH, assuming your clutch has zero slippage
70 x 3.55 div/by .006 div/by 13.5 = 3069 RPM
~example~
if you run a 27"-tall tire with 3.55:1 gears, it'll require approx 3100 RPM to run 70 MPH, assuming your clutch has zero slippage
70 x 3.55 div/by .006 div/by 13.5 = 3069 RPM
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Change Rear End Gears??
Your problem is with the close ratio transmission - the close ratio M21 (Borg Warner ST-10) in 1974 had a 2.43:1 first gear ratio which is too high for great initial acceleration; if you change to a lower 4.11 rear end ratio, you get the acceleration, but it really hurts at the gas pump!
A better solution is to get a wide ratio Borg Warner ST-10 (Richmond Gear now makes them, and are sold by Summit Racing) in which you can select a lower 2.64, 2.88, or 3.42 first gear ratio. Since you never mentioned your rear end ratio, you'll want to get ~10:1 total first gear multiplication. All three of these transmissions still have a 1:1 4th gear ratio so your cruise rpm will not change.
For example, if your close ratio has a 2.43 first gear, a 4.11 rear end will give you a total first gear multiplication of 10:1; if your rear end ratio is ~3.36, get the 2.88 first gear ratio; lastly, if you have a 3.08 rear end ratio, get the 3.42 first gear ratio (10.5 total).
A better solution is to get a wide ratio Borg Warner ST-10 (Richmond Gear now makes them, and are sold by Summit Racing) in which you can select a lower 2.64, 2.88, or 3.42 first gear ratio. Since you never mentioned your rear end ratio, you'll want to get ~10:1 total first gear multiplication. All three of these transmissions still have a 1:1 4th gear ratio so your cruise rpm will not change.
For example, if your close ratio has a 2.43 first gear, a 4.11 rear end will give you a total first gear multiplication of 10:1; if your rear end ratio is ~3.36, get the 2.88 first gear ratio; lastly, if you have a 3.08 rear end ratio, get the 3.42 first gear ratio (10.5 total).
