yellow71

I have a stock 71 LT-1 with M-20 and 3.70 rearend. I would like to lower the rpm's at highway speeds and would rather to try a rearend swap before going to the expense and hassle of a transmission change. I have read threads on the best allaround rearends for the M-21 and M-22 trannys for highway (3.55 or 3.36) but have'nt seen recommendations for the M-20. Also where is the best place to buy the new rearend setup. I live in Louisville.

Thanks
Neil

OHSIXX

I am on the other end of the spectrum (3.08) and debating weather to go with the 3.36 or 3.55. Like you, I don't want the high RPMs on the highway.

mrvette

hehe, like anything else it's a compromise, UNLESS you have an overdrive.....at 80 mph a 336 will put you at near 4 grand, that's a lot for hour after hour, but if you in a metropolitan area, and don't sustain that for over a few minits if that....it's good enough...at 336,
but considering most street tires will easily light off even with a close ratio muncie, let along wide ratio...hell, even a 308 rear is easy to spin wheels with a stick, let alone with an automatic....

so depending your purpose, you just don't NEED further lo end take off response....depends on YOUR driving habits....

I kept my 336 and with the auto overdrive 700 tranny my highway RPMs dropped to 2500 at 80.....fine range, runs all day at that speeds...

GENE

stingr69

M-20, M-21, or M-22 all have a 1:1 top gear so it makes no difference at highway speed.

3.36 will lower the RPM enough that you can feel it but it will be obviously slower out of the hole. I would not go any numericaly lower with the LT-1 IMO.

-Mark.

Gordonm

In my LT1 I originally had 3.55s with a M20. I went to 4.11s and then to 3.70. I liked the 3.70 gear the best. I now have a Tremec with the 3.70 gear and it is great. Going from a 3.70 to a 3.36 will drop your cruise rpm by about 10%. I dropped my cruise rpm by 32% by going to a Tremec.

Surfer69

Just for reference my 3:36 runs 70 at 3100 and 80 around 3500.

Ol Blue

Yellow71
Try Ikerd's in Bedford Indiana for a differential. He's been doing them for years. http://www.ikerds.com/
Ol Blue

isosceles

if there is ANY chance you'll be getting an OD in the future, don't change the ratios.

I went from a 3.70 down to a 3.36 in Spring of last year when I had the L48 and TH350 trans. I was taching over 3000 at 60. The 3.36's lowered it a couple 100.

While I enjoyed lower RPM while I had the TH350, I ended up breaking that trans and replacing it with a 200r4 (OD) tranny over the summer. Way sooner than I expected.
Now, especially with the 383, I wish I had my 3.70's back. Someday I will.

Oh yeah, where to buy? I got mine from Van Steel. I have at least 3K miles on it with 30 drag runs, 15 behind the 383. It's quiet and smooth.

stingry

I have a M20 and 3:36 rear end. I am happy with it but would be even happier if I had a 5 or six speed in there instead of the muncie. 65mph = 3000 rpm. With some good soundproofing and a reasonably quiet exhaust it is very comfortable on the highway.

You can get OD gear kits for the muncie if you high ratio rear end like yours. Do a search on muncie overdrive gear sets and 5 speed muncie. I don't know if it is the best option given the ruggedness of a Keisler 5/6 speed but at least it gives you options.

From what I have costed you can not get an O/D system in the car for less then $2500. Exception is if you go to an OD auto like the 700R.

Fevre

isosceles

Get yourself a higher stall, something in the 2400-2800 range will make it feel like you have a higher gear, with that high duration cam you are engaging the t/c before the eng gets into its sweet spot for torq.

LemansBlue68

I thought all the LT-1's had M21 close ratio trans's. Could be wrong though. Change from 3.70 to 3.36 will drop your 4th gear revs about 10%. You'll still have a respectable 1st gear launch because of the M20's 2:52 1st gear vs. the M21's 2.20. This is roughly quivalent to an M21 and a 3.70 rear end in 1st but your 4th gear revs would be lower with the M20 and 3.36.

I considered going to an M20 because of the lower 1st gear, but since I had to have my rearend rebuilt, I opted to stay with the M21 and increase my rear gear from 3.36 to 3.70. My car will be mostly for blasting around town with not so much highway driving.

Your proposal sounds like a nice combo in lieu of going to a 5 speed.

mstanton

M21 was an option on LT-1s just like all other cars. Most LT-1s came with M21 but many did not. I had a 70 LT-1 with M20 and 3.36 rear. I liked that trans/rear combo better than the M21 with 3.70 that I have in my 71 LT-1. The M20 and 3.36 is good off the line and comfortable at highway speeds.

shoptek

Mine too!!

AGVI

Be a man and go for 4.11's, after all you have a 5 speed!
Come on, you know you want to!!!!!!!!!!!!

GDaina

3:55's here, 2800 at 70....can't go 80, or even 75, else I be feedin' the State Highway Patrol's tickey pool....

87bob

Ikerts in Bedford Indiana will help you out with the rear end parts. I talked to him at World of Wheels in Indy a couple of weeks ago about the very same thing in the wifes 79.

David Ey

anybody have a used one needing rebuild for sale?
ratio unimportant

68/70Vette

Yes, with a 3.70:1 you're really causing a lot of engine wear just to drive on the freeway. If you want to spend a lot of money on a performance engine, it's a shame to just grind it down for a mundane drive down the freeway.

I've had a 427 BB piece part built L88 with a 3:08.1 rear end and.. a Richmond 5 speed. The Richmond has an extra low first gear (an underdrive first gear). I think that in my Richmond first gear, my gear ratios were effectively the same as a stock Muncie and a 4:11.1 rear end! In other words,.. dramatic launch even with a 3.08:1 rear end. A friend of mine, with a really built 454 in a 1967 Corvette installed a Richmond and.. a 2.78:1 rear end!!!(He wanted a really high top end speed at engine redline) These numerically low gear ratios mean that for highway cruising you're not stressing your engine. I think my 1969 would do about 73 mph at 2000 rpm. I have a 1970 Coupe with a 3:08.1 rear end, but I still am using the Muncie. Still the 1970 does this 73 (?) mph thing at 2000 rpm.

I think if you want performance, and also the relaxed feeling that you're just not grinding the *hit out of your very expensive engine on the freeway, you really...really....really...have to go to a five speed transmission. I like the Richmond because in top gear it's one to one. (i.e. straight through - no transmission power losses.) However, the Richmond is tough to shift. I know they get a little looser with wear...

I can use very light finger pressures and palm pressures to shift my Muncie. The Richmond takes more pressure. However, I believe that it is a massively strong transmission. ( I like it's siren like sound in third gear.)

Most people today seem to be buying Keislers. These are five speeds with overdrive 5th gears. (The Richmond 5 speed has an underdrive 1st gear.) I've never read any comparison between the two transmissions.
The Richmond will fit in a C3 Corvette, even with a fixed crossmember. However...it is a very tight fit.

Anyhow, Bottom Line...Get a Five Speed. Get low speed snap the neck, spin-the-wheels performance and then cruise on the interstate at a low idle like rpm!

OHSIXX

I sure am glad to have read your comments. Your "bottom line" is exactly what I want. The Richmond sounds like it is for me rather than going with a lower gear. Thanks for straightening me out.

copter

The LT-1 worked best with a lot of gear.Thats why GM put standard 373's in the Z/28.