moranstl

I have a 75 coupe with original 350 and 400 automatic tranny. I'm getting the rear end rebuilt. the original 3:08 gears in it were a pooch. Question is ...should I go with 3:36 or 3:55 gear. I live in a rural area and do a lot of back highway cruising. Any thoughts and opinions would be appreciated.

USA-OSC

I think for back highway cruising you would appreciate the 336 more than the 355. I have a stock 73 454 m21 with 355 and I feel like I'm running high rpm's most of the time while cruising.

bluedawg

I vote 3.7 you'll notice the gain and it won't be that many more rpms on the highway.

toobroketoretire

If you are only interested in cruising I would stay with the 3.08's or even go with 2.72's to keep the cruising rpm real low. The low 3.73's are okay but ONLY if you have an overdrive transmission.

bashcraft

Where would you get a 2.72 ring and pinion for a 75 corvette?

jim-81

This is the exact dilemma that I have. Currently in (my 81) I have 2.87 gears and I want to change that out to either 3.36 or 3.54. So I will be watching this thread. I want better acceleration but still want to drive it on the highway. Sorry - not trying to hijack this thread.

jim-81

And who would want a 2.72? good grief!

COOLTED

I have 3.55 in the '79 and It's great on the highway, but interstate speeds are a bit uncomfortable. last time on the highway I got 24 mpg. Interstate at 70-75 I got 16.

bashcraft

I would. I'm running a 3.08 now and my 502 could easily pull a 2.72 gear.

jim-81

What's the difference between highway and interstate? (in your definition)

gkull

how did it drive with 3.08? I work on a vette that the owner went from 308 to 3.36 and the owner loves it now. He has a 325 hp summit racing crate motor. Even though it is a roller cammed motor it is relatively low rpm like your 75. It is just a fun driver and cruiser.

daustin

I've got 3.36 in my '68 427 car and i wouldn't have anything else with the Muncie 4 speed. Good RPM's on the highway and enough gear to motivate it pretty well off the line.

Don

diehrd

I would go 3.55 with a stock motor because if you ever go to o/d its the sweat spot gear for driving and even if you do not go O/D it will give you down low pulling power you will really enjoy.

My guess is you want the 3.55 but are worried about highway rpm

This may help you decide

http://chevellestuff.net/tools/rpm_calculator.htm

F22

My vote is for the 3:55. I'm running a mildly mod'd L48, and for me, it's right there in the middle. Not too low, but not too high. Saturday, I ran it up the hill at 85-90+, for half an hour, running about 3,500 RPM.

ddawson

I have a 3.36 and it's right at 3K at 70 MPH. A 3.55 isn't going to change it much.

I do find my self reaching for a 5th gear though. Just a habit from my other cars.

@ 70MPH
3.08 1880
3.36 3037
3.55 3208
3.73 3371

AboveTheLogic

My '79 with 3.36s and a 4-speed does right around 2,800-3,000 rpm at 65-70 mph with 275/60/15 tires. You can always swap taller or shorter tires down the road to bring down your RPMs. I like the 275s for highway driving, thinking of finding a used set of wheels to mount 245s on for the days that I'll want better acceleration in exchange for higher freeway rpms.

Having said all that, a T56 with 4.11s is in the works for 2016

REELAV8R

trackdogg2 is changing mine out from a 3.08 to a 3.55. What tipped me in the direction of the 3.55 rather than the 3.36 was the though of putting in a 4 speed down the road.
I figure I'll run it a couple years with the 3.55 and the TH350. then end up with a 200R4 which will complement the 3.55 ratio very well.

COOLTED

The highways around here are 60 or 65 mph, the interstate is 75.

mrvette

I bought my '72 , old show car in '95 stock L48/Muncie/336 rear, put on later C4 wheels/tires almost immediately, HUGE improvement in handling, but did not fill the wheel wells right.....

In the Wash DC region, you can do 80 mph for maybe 5 minits if that, you run into cops or traffic, so no need for any o/drive.....Moved to Florida in '97, and going up/down I 95, I doing what I thought was 80, according to the speedo, anyway, and turning what the tach said was 4 grand.....I forget if I still had 40 series tires on it or had switch to 50 series by then......

at any rate, down here I switch to an auto o/drive trans, to stay with a stickshift would have run about 5 grand, instead of ~1500, age 71 now anyway....age 53 back then.....

it's a 200 4r now, with FI on the car, from up north yet, thing got 24 mpg on I 95 at ~80 mph and 4k rpm.....

SO, some years ago, I got into a 'street race' with some fellow over the local 3.5 mile bridge, and posted somewhere about it, and so I had thought I was doing like 150 mph......some fellow called me on it, and so I got to thinking, and I respected his opinion....

went to some store here, and bought a GPS......fired it up across the same bridge, speedo totally off, forget the tire size/diameter/ratio/rpm calculator sites, I believe the GPS which proves them RONG.....got no clue how so much can be proven RONG, but it was, and I ran to many sites and plugged in the numbers, and they ALL said it was X at Y rpm.....bullcrap.....

I finally fixed the trans output driven gearset so the speedo read same as GPS, now here is the funny part.....they totally agreed up to 70 mph, right on the money....at 80, it was really only 78 by GPS..... at 100 indicated, it was really only 94 mph......shocking to ME, but I have to assume GM built that into the design.....speedo seems fine, no wavering, cable been reworked, very consistent......

So what about the tach?? Me being an old ET here, I have a Tektronics scope, and a digital signal generator.....so I line synch the scope and set the generator at 60 hz, right on the money......I take a high power transistor and key it into the HEI tach post which is where the black module fires the coil at.....so checked the tach at various 'rpm/frequencies....600 idle speed, right on, 2k right on 2500 is a little over reading, by 4k the tach was about 600 rpm high and at 5k the tach was WAY the hell over indicating....again, deliberate????

I have stayed with the 336 ratio it's fine with the 200 trans, works like a charm, and been very consistent some years later I took the GPS on a trip, and everything was fine.....up to 80......

Gale Banks 80'

Despite what catalogs say the 3.73 and 3.55 are easy to come by and most other ratios are not. You might find that Your choices are pretty slim.