bj1k

I just got my 72 on the road today and it runs great with the close ratio M-21 4 speed and 3:73 rear gears but too much engine rev in fourth gear for me . More power than I need and would like to go with lower engine rpm's in fourth and better gas mileage. Just had the rear end gone through by a professional rear end specialist and with only 52k. miles it just needed new seals . I will probably keep this rear but I am considering buying a 3:08 rear and was wondering if anyone is running the close ratio 4 speed M-21 with the 3:08 rear and how you like it. I looked up the 72 specs and it looks like they came from the factory with this combination or the 3:73's and I know it should be great on the highway with 3:08's but concerned it might be a strain on the clutch starting out on hills. Anything that I should be concerned with ?

68/70Vette

You really should think about a 5 speed. I have a 3.73:1 rear end gear ratio in my 68 and I've installed a TKO600 five speed. Cruise at 70 mph at about 2000 rpm's with the five speed. Looking at the interior, there's no way to know there's a 5 speed in the car. The TKO600 uses a stock like looking duplicate shift lever. I kept the Mucie 4 speed shift pattern on the shift console, so even that doesn't indicate a 5 speed.

If you keep your Muncie and change the 3.73:1 to a 3.08:1 differential, you'll find that the engine will struggle in first gear if you have a performance cam shaft.


The TKO is a modern trannie and shifts really smoothly.

zwede

Mine was originally an M20 3.08 and it sucked. Went to a TKO with a 3.27:1 1st gear and the 3.08's still sucked. Now have 3.73's and it's perfect. I love all the gear ratios, the rpms are just right.

bj1k

I will have to wait for a lottery win for the 5 speed . Right now I am just looking for an economical way to get the revs down a bit and increase the gas mileage. Right now it seems like there should be one more higher gears to shift to. This is a great setup if you are into street racing though.

grancuda

What size tires you running?

3JsVette

3:36s maybe a compromise. With a 27" tall tire that should get you under 3000 RPMs at 70MPH. A 5 speed would be the best solution though but sounds like it's not in the budget.

Danish Shark

308 don't work with a manual.
Even with a wide ratio M20 it sucks. First gear is way to tall.

GrandSportC3

I have 3.08's in my '68.. 2700 rpm at 70 mph.. great highway gear but not the best for smoking tires..

cagotzmann

What kind of mileage are you looking for ?

I have a 78 with a ZZ383 and a T56 Close ratio 2:66 & 3.08. Highway mileage is 27.8MPG (23.4 US) thats with a double over drive. Street driving with a 3.08 & 2.66 first is not as bad I thought it would be. With only a 1:1 max top end I am not sure you will see much in fuel saving.

@ 65 MPH you are running around 2550 RPM 3.08

vs 3.73 @ 3100 RPM.

But if the long term ownership is highway driving the best way is with and overdrive transmission. Keep the money in your pocket and save for a 5 or 6 speed if you can.

hwcoop

most of my driving is hiway and i figure the 454 will provide enough torque to solve my tire smoking issues
with my 3.70's i can watch the gas gauge go down LOL but wow it hauls a$$

scrappy76

This is where I am at. I installed a 454 in my vette. It has 3:73 gears in it now. I am trying to decide between 3:08s and 3:36s to get the highway rpms down. I only want to do it once. And I have no interest in swapping in a 5 speed or a 700r4. Local guy has a clean diff with posi for sale with 3:08s. Only wants $375 for it.

65GGvert

I have 3:08 in one of my manual cars. It turns 2600 rpm's at 70 mph and I'm perfectly happy with it. There is no bogging or strain in first and in fact the car will run faster (mph) in each gear, so you are changing gears at higher speed if you use the same rpm shift. It sounds much better at 70 because of the lower rpm. I no longer spin tires or race at stop lights, so I really like the choice.

LS4 PILOT

M21 and 3:73 is a beautiful combo, the shift spacing is sweet and keeps the SB at just the right pace for spirited driving. But , I can see it might get bothersome for extended periods at say 60-70 mph.

My current 454 has the M21 2:20 Low. W/3:36 axle which I find to be a really nice compromise between performance and low RPM driving.

I driven couple BB. Cars with3:08 axles ... Liked it a lot , but scared I get too many tickets.....those cars would run 130 so easy . I was always wanting to run 80 -100...lol

I have had a 3:70 and 4:11 with BB ...M21 ...the drone of the BB and 4:11 sounded like a B24 bomber at 65 mph...I drove that thing to work 40 miles each way ..LOL


With M21 . 3:36
....though First gear is pretty fast ..LOL.......I do most of my driving local in 1-2 gear . 3-4 out on the highways .

bj1k

What transmission are you running ? I have a close ratio with 3:73 rears and skip second gear and go straight to third . Second gear is useless.

bj1k

It's not so much the mileage that I am concerned about as the wear and tear on the new engine because most of my driving is on open road and I'm not into racing and burning off the tires. (Old guy talking here )

72ls5fla

I think the 3:36 will be a great compromise for you.

They will still give some bottom end with the SB and will for sure keep the RPMs at a decent spot when driving on the HWY. they are a decent match for shifts with a M21.

With 3.08s you can run 80MPH at 3000 RPM (stock tire)
the 3.36 gears will keep you right around 70-72 MPH same RPM

as for gas mileage - its an old Vette. its gonna suck no matter what you do unless you spend lots of $$$$ investing in a transmission and building more efficiency into the engine.

cagotzmann

I guess then you would like a simple 1800-2200 RPM range for highway driving. @ 3100 vs 2550 this will help

A 2010 C6 GS auto car ratio in forth is 2.73 (Rear) * 1.15 4th = 3.14
A 2010 C6 GS 6 speed ratio in forth is 3.42 (Rear) * 1 4th = 3.42

Your car ratio in 4th 3.73 Rear * 1 4th = 3.73

Changing the diff to match new corvette's would help. So a 3.08 rear in you car would put it similar to a C6 auto. But where it will hurt is in the lower end your engine will work harder to get the car moving. The new cars have a 4.03(auto) 1st or 2.97 (6 speed) First.

Its the 5th and 6th gear that saves these engines.

My car feels most relaxed on the highway in 5th. Not reving high but yet has enough to slowly accelerate if required. @ 65 MPH I am running 2000 RPM in 5th. 6th is great if the roads are flat and open. Doesn't require much throttle to keep the car cruising

My Ratio's

1st 2.66 * 3.08 = 8.19 ( Poor performance ) Need torque to move
2nd 1.78 * 3.08 = 5.48 Great throttle response in this gear
3rd 1.3 * 3.08 = 4 sluggish at low RPM needs 2500+
4th 1 * 3.08 = 3.08 sluggish below 3500
5th .8 * 3.08 = 2.46 (note how this matches a C6)
6th .63 * 3.08 = 1.94 (note how this matches a C6)

C6 manual
1st 2.97 * 3.42 = 10.16 ( recommended by most ~ 10)
2nd 2.07 * 3.42 = 7.08
3rd 1.45 * 3.42 = 4.96
4th 1 * 3.42 = 3.42
5th .71 * 3.42 = 2.43
6th .56 * 3.42 = 1.92


Your car

1st 2.20 * 3.73 = 8.2 Same as my car poor compared to new cars
2nd 1.64 * 3.73 = 6.12 Similar but slighly less performance to a C6
3rd 1.27 * 3.73 = 4.74 like a C6 Almost
4th 1 * 3.73 = 3.73 like a C6 but better acceleration

now with 4.11's

it would feel like a C6 1st - 3rd gear pull a C6 in 4th and then flat out due to no RPM's left.

1st 2.2 * 4.11 = 9.04
2nd 1.64 * 4.11 = 6.74
3rd 1.27 * 4.11 = 5.22
4th 1 * 4.11 = 4.11

Your car with 3.08

1st 2.20 * 3.08 = 6.78 Will be hard on the clutch to get moving
2nd 1.64 * 3.08 = 5.05
3rd 1.27 * 3.08 = 3.91
4th 1 * 3.08 = 3.08

Here is what I expect your experience will be like moving to a 3.08

1st hard on the clutch
2nd will feel like driving in 3rd
3rd will feel like a auto downshifting from 4th
4th will feel like driving 55 MPH while doing 65


Try to find a car that already has what you are thinking about doing and drive it. See if you like it.

qwank

how many miles do you drive it a year? If you can't afford a 5 speed right now it might just be cheaper in the long run to leave it as is and pay a little extra for fuel.

65GGvert

M20, and 2nd gear works great with the shift, far from useless. They wind much longer before you need to shift, but the 327 has plenty of torque if you shift at a lower rpm.

bj1k

It's definitely not being able to afford the 5 speed it's not willing to spend the money on it. Besides what's different ? Am I wrong thinking that all top gears are 1 to 1 ratio ( straight through ) ? How does that give you lower rpm's without changing the rear ?