dazmuth

My 1973 454 is about as original as it comes, including the original close ratio 4 speed transmission and its 3.55:1 rear end. It's easy to see why later Vettes had 5, 6, or 7 speeds, because 4 just ain't enough. It's screaming on the highway and slurps petrol like it's free. Since gas isn't actually free and I like not having to shout to my passenger when I'm driving, I'd like to tweak the rear end to something more reasonable. I'm not burning rubber anymore so let's be reasonable.

Anybody out there put in a new rear end on your 454 C3? There are several different after market rear ends at my disposal, notably a 3.08:1 and 3.36:1, and I'd like to hear what ya'll have to say about it.

I'm not entirely opposed to dropping a Tremec 6 speed into it either... anything to improve on the 13mpg. But I'd rather keep the 4speed if a revised rear end is doable.

Anybody changed their rear end? (insert Kardashian joke here)

stingr69

I would not change the rear gear so quickly. You will loose a lot of your big block "grunt" if you get the highway revs down much at all. Transmission swap would be my choice first and it would not hurt the resale value at all in my opinion. Might even raise it.

bradleyb66

My vote would be for the tremec 6 speed if you're concerned with mileage. For comparison, going from a 3.55 to a 3.08 rear ratio would drop your 70mph rpms from 3092 to 2683 (assuming 27" tires). Putting in a 6-speed gearbox drops you from 3092 to 1948 (assuming 0.63 6th gear ratio of the tremec). And 5th gear on the tremec would be 2474 at 70mph.

suprspooky

If you're not worried about the push you into the seat effect, I'd say trying a different rear gear is way less money than a trans change (and I think the labor is less painful). On the other hand the tranny change gets you the best of both worlds. I have a 4.11 rear now and just picked up a rolling Chassis that has a 3.08, I plan on swapping them this spring to see what it'll do in 1/4 with the 3.08's. (I do have a fair amount of torque to work with vs. a stock 454)
The 4.11's are fun for stomping on the gas but 3500 on the freeway is annoyingly loud with side pipes, someday I'll move to a tremec but it ain't cheap and if you don't have access to a lift, well have fun.

I would love to have a 3.36, nice compromise in my opinion.

GTR1999

If you truly have an M21 CR trans then a 308-336 isn't going to give very good 1st gear launch. It will work but a M20 WR would be better. Many BB cars came with M20 and 308 and the BB torque handles it.

I would not expect to much improvement in MPG with keeping that M21 with any ratio. You will see it with an OD trans and 355's will work with an OD but the only MPG improvement will be on constant speed use, around town you might not see what you want.

Yes an OD trans will cut the RPM down and has less effect on the engine but how much are you going to drive the car? If 10,20,30k miles a year it might be worth it. I kept the original M20 and TH400 in my cars since I don't put more then a 1k miles a year on it. It cost far less to spend $3-$4 per gallon to drive the car then it would cost to swap to a OD and look for the payback period.

A 73 vette diff has soft axles so it's original you might want to check those since if they are wearing down they will crash into the diff housing and cause you some issues. The clutches used were not the best either.

I am curious to what you say are aftermarket diff's? Are they just a rebuilt iron or non original iron diff's like a 9" converison or something else?

2mnyvets

I bought a 73 BBC M21 corvette that wasn't running. Like you, I wanted to keep the 4 spd and I went with the 3.08s and an Autogear transmission. I should finish the frame off in 2-3 months.

What you do should depend on how original you want to keep the car and how much money you are willing to spend. The 3.08s are the cheapest option to reducing RPMs and noise. The 6 spd is the most elegant and expensive option but it requires cutting up the car and it is more work to put in. In between you have a number of other options.
I would install the 3.08s. If you are happy with the acceleration now and unhappy with the acceleration after you install 3.08s, consider switching to an M20. The overall ratios(M21 w/3.55 vs M20 w/3.08) are nearly identical for first through third gear (see attachment). It is a bolt-in and you should be able to sell your M21 for the same as you pay for an M20, so all you lose is the labor to switch the transmissions. If your trans needs a rebuild, M20 gears fit in the M21 box and you keep your numbers matching case. (Vin numbers are stamped into the case).
I chose the Autogear M22Z. My trans needed a complete rebuild and a number of gears. The Autogear was cost effective and gave me better gear ratio options. It is a direct, bolt-in replacement. It eliminated a number of the Muncie weaknesses. Autogear offers a number of different ratios, so check them out. I put my numbers matching trans on the shelf.

If you are looking for fuel mileage, consider an injection system. To GTR's point, you probably cannot justify any upgrade on fuel mileage alone, but if/when you have carb issues the additional cost of FI may be worth it and will probably get you better mileage than the 6 spd.

JBL Specialty

My first Corvette was a 68 coupe with a 427-390 m21 and a 3.08 rear. At 80mph it was turning 3000 rpm. At highway speeds it would get 20 mpg. It also ran very good for its ratio. A guy I worked with had a super cobra jet mustang with the drag pack. We raced once at local drag strip . I ran a 13.86 and beat him so bad, I could see him in the rear view mirror at the finish line. Not bad for the economy rear.