Rear Gears
Thread Starter
Heel & Toe
Joined: Oct 2024
Posts: 24
Likes: 2
From: Puyallup, Wa
Rear Gears
My 70 LT-1 currently has 4.11 gears. Originally the car had 3.70 gears in it. I don't really don't like how high the RPM is at 40-60 mph. I think 40 mph is 2200 rpm and 45 to 50 is 2500 rpm. I won't be drag racing anyone, in fact keeping up with Camaro's, and Mustang's of today is not realistic...Tesla's forget about it. Having the fastest car wasn't my intent. I always wanted a badass car from this ERA. I have that. My roundabout question is putting 3.70 or even 3.55 lower enough lower the RPM that much. I'm hoping 3.70 would lower it enough to be worth it.
Race Director
Joined: Mar 2016
Posts: 14,320
Likes: 8,124
From: Napa Valley California
I would first think about what your rpm goal is for what you’re planning to use the car for now.
If you plan on cruising on the highways at 65-70 you might want to go to a 3:36 gear which would still put you at 3000 rpm in 4th gear.
This is assuming you currently have a 4 spd manual in it.
You might be better off leaving the 4:11’s in the rear and do a 5 spd TKX with a 0.64 5th gear.
If you have an auto then there are several overdrive automatic transmissions you can go with.
A lot of this really depends on all if any engine modifications you’ve made for drag racing as the camshaft can play a big part in performance at cruising speed and rpm.
BB or SBC?
I’m currently running a 496 with a TKO600 5 speed, 0.82 5th gear and 3.08 rear gears.
At 70 mph I’ll be cruising at 2300 rpm in 5th gear.
If you plan on cruising on the highways at 65-70 you might want to go to a 3:36 gear which would still put you at 3000 rpm in 4th gear.
This is assuming you currently have a 4 spd manual in it.
You might be better off leaving the 4:11’s in the rear and do a 5 spd TKX with a 0.64 5th gear.
If you have an auto then there are several overdrive automatic transmissions you can go with.
A lot of this really depends on all if any engine modifications you’ve made for drag racing as the camshaft can play a big part in performance at cruising speed and rpm.
BB or SBC?
I’m currently running a 496 with a TKO600 5 speed, 0.82 5th gear and 3.08 rear gears.
At 70 mph I’ll be cruising at 2300 rpm in 5th gear.
Drifting
Joined: Nov 2013
Posts: 1,570
Likes: 485
From: Loud, Raw and Dangerous 1968 327 4S in Southern California
My car has 3.08 gears (M20 trans/225 60 R15 tires) and it is just slightly too tall especially if on a steep hill or the engine is cold. Wonderful on the highway at 70 MPH and 2800 RPM's. If I was going to change the diff ratio, I would likely go to 3.36 but not worth it at this point. Car is just a weekend driver and fast enough for me.
https://spicerparts.com/calculators/...rpm-calculator
https://spicerparts.com/calculators/...rpm-calculator
Burning Brakes
Joined: Jan 2006
Posts: 1,072
Likes: 331
From: Clear Lake Shores Texas
2016 C3 of Year Finalist
If you have the M21 close ratio transmission I wouldn’t go lower than a 3.70. The first gear ratio makes the car a little difficult to launch with the 3:55. If you have the M20 the 3.55 gear was an option because of the different gear ratio. I have the 3.70 in my 72 LT-1 and sometimes wish had the 4.11. Just my opinion.
Drifting
Joined: Jun 2007
Posts: 1,692
Likes: 157
From: Miami, FL
I was just going through this with my newly acquired 70 BB. I have the M21 trans and 3.70 gears. My RPMs are half my MPH, so at 60 I'm doing 3000 RPM. I discussed going down to a 3.36 or even lower but was talked out of it by my Corvette shop owner as it is a desirable combination.
With yours being a LT-1, and a 70 at that, if it is original engine I would be inclined to match the rear end to original. It can handle the revs.!! And it should be driven to them often.
I'm going to keep the 3.70
With yours being a LT-1, and a 70 at that, if it is original engine I would be inclined to match the rear end to original. It can handle the revs.!! And it should be driven to them often.
I'm going to keep the 3.70
Race Director
Joined: Mar 2016
Posts: 14,320
Likes: 8,124
From: Napa Valley California
I was just going through this with my newly acquired 70 BB. I have the M21 trans and 3.70 gears. My RPMs are half my MPH, so at 60 I'm doing 3000 RPM. I discussed going down to a 3.36 or even lower but was talked out of it by my Corvette shop owner as it is a desirable combination.
With yours being a LT-1, and a 70 at that, if it is original engine I would be inclined to match the rear end to original. It can handle the revs.!! And it should be driven to them often.
I'm going to keep the 3.70
With yours being a LT-1, and a 70 at that, if it is original engine I would be inclined to match the rear end to original. It can handle the revs.!! And it should be driven to them often.
I'm going to keep the 3.70
Thats what was happening to me on warm days.
I always found myself at 55 mph and watching the temp gauge anytime I was on the highway.
55 mph on California highways is a perfect way to get rear ended.
With a BB or any SBC with some torque built into it you can easily get by with a 8.5 first gear ratio overall and unless you are drag racing you won’t loose much on take off.
My TKO has a 2.87 first gear ratio and coupled with my 3.08 rear gears puts me at an 8.8 first gear ratio.
You really have to look at the overall ratios of all gears with factoring in the rear gears, plus camshaft, torque, hp and where the engine produces the torque.
Tech Contributor
Joined: Aug 1999
Posts: 15,154
Likes: 3,959
From: Connecticut, USA
My 70 LT-1 currently has 4.11 gears. Originally the car had 3.70 gears in it. I don't really don't like how high the RPM is at 40-60 mph. I think 40 mph is 2200 rpm and 45 to 50 is 2500 rpm. I won't be drag racing anyone, in fact keeping up with Camaro's, and Mustang's of today is not realistic...Tesla's forget about it. Having the fastest car wasn't my intent. I always wanted a badass car from this ERA. I have that. My roundabout question is putting 3.70 or even 3.55 lower enough lower the RPM that much. I'm hoping 3.70 would lower it enough to be worth it.
However, if you want to drop the gear to 370 from 411 you have to see what you have inside first. Is it a GM 411 setup, it is an older Tom's 411 thick setup, is it the current Chinese knock off of Tom's 411 Thick??
Can it be done, yes. Cost depends on what you have, what you need and who does the work and the level of the build. That comes to between $800 and $3500 today.
Thread Starter
Heel & Toe
Joined: Oct 2024
Posts: 24
Likes: 2
From: Puyallup, Wa
My intention was to just drive it cruising around and car shows. The engine was rebuilt several years ago but all matching numbers and correct date codes for a CTU stamped engine. Big oil pan so no power steering, I do have a modern equivalent holley carb on it with the whats probably the original carb rebuilt in a box date coded to dec 69. The original exhaust manifolds were put back on after having side exhaust for a while. The only thing I'm not sure of is the rear end. When going over the codes the seller thinks it was from a 1973 corvette but I can't make out the numbers. I do know that it does have the 4.11s. They kept everything, I have the original water pump with the gaskets they scraped off. At least the more recent owners kept all the fuel receipts. I got all the trophies it won in car shows. It was originally sold in Canada and have alot of Canadian documentation. All the documents barely fit in the car when it was shipped. It's been fun learning this car.
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Melting Slicks
Joined: Oct 2001
Posts: 2,064
Likes: 1,382
From: Halifax, Nova Scotia, Canada
If I may give my opinion.
My configuration was LT-1, Muncie M21, 3:70, with added GM side exhaust.
What I did not like was, (Which was great when I was young)
- High rpm while highway cruising, 3,000 rpm @ 70
- The 2:20 first gear of the M21
- Noise from the side exhaust at highway speeds
My solution was a Tremec TKO600 .64 od (Now)
- Highway, 70 mph, 2,000 rpm
- First gear 2:87
- 80% reduction in exhaust noise on highway.
- Substantial increase in fuel milage.
A 5-speed solves all your problems with one change. Major downside is the cost.
I have never regretted spending the money on the 5 speed. Installed in 2006.
Barry
My configuration was LT-1, Muncie M21, 3:70, with added GM side exhaust.
What I did not like was, (Which was great when I was young)
- High rpm while highway cruising, 3,000 rpm @ 70
- The 2:20 first gear of the M21
- Noise from the side exhaust at highway speeds
My solution was a Tremec TKO600 .64 od (Now)
- Highway, 70 mph, 2,000 rpm
- First gear 2:87
- 80% reduction in exhaust noise on highway.
- Substantial increase in fuel milage.
A 5-speed solves all your problems with one change. Major downside is the cost.
I have never regretted spending the money on the 5 speed. Installed in 2006.
Barry
Le Mans Master
Joined: Aug 2005
Posts: 6,521
Likes: 1,956
From: Temple Terrace Florida
Another vote for the 5 speed with a .62 overdrive. I installed it in my 66 also with a 4:11 rear. In overdrive I am at about 2250 RPMs at 70 MPH. I also have side pipes. Without the overdrive, I could not hear myself think at 70. The noise was way too much, especially over a prolonged period of time. My 68 has a M-21 and a 3:70 rear. As a kid I did not mind the high RPMs on the highway, now that I am older, I take the 66 with overdrive more frequently on the highway. Jerry