Cali,68,L-79

I currently have 370 in the rear mated to a M-21CR and original L-79 motor. Not happy with highway drivability and not willing to spend $4,000.00 on 5-speed. Currently have appointment for Tuesday for Pro-Gear in San Diego, CA to install Richmond 3.08's and rehab the entire rear. (I'll save the 3.70's if I ever sell her)
I need either the RPM calculator or your RPM's at 75mph. Tires are 255/60/15's. I'll lose some off-line but with the close ratio tranny it won't kill me and I don't race anymore so highway friendly means more to me than ***** to the wall performance. Wish to join the long distance trips with my Vette club.

sb69coupe

david, I'm not so sure that you're going to like what you end up with. The combo of the M-21 and 3.08 gears would be sluggish off the line even with a very stump-pulling torquey engine. Couple this with an L-79 or an engine modded further toward higher RPM power at the expense of low end torque, and you'll end up with a powertrain combination that is not very drivable from stoplight to stoplight.

I'm pretty sure that the lowest numeric gear that you could get with an M-21 was 3:36. Most of the L-46 and L-82 cars came with 3.55 gears standard with the M-20, and 3.70 with the M-21.

Quinn_Seipp

I would be interested in this as well. :lurk:

Double Aught

:withstupid: Think about this before you spend big bucks. An engine that has it's engine powerband in the mid to upper rpm level, needs the lower gear ratio to get there. That is why alot of ZR-1 owners put in the 4:10 gear. It gets their engine into it's powerband that much faster. You need the OD gear in the tranny. Save your money for a 5spd conversion. There was a big thread on converting to a five or six speed tranny in a C3 just the other day. Re-read it, if you haven't already. Don't go to a 3:08 gear. You will be very disappointed.

71coupe

The 3.08's will work with a BB or Rich 5 speed, but may be too much for a SB. If I was in CA I'd let you drive my car (which is still in calif) It has 3.36 gears & I think you would like it. I usually drive between 75-80 mph and it's not that bad (around 3200 rpm @ 75).

If you want to figure out engine speed per MPH, use this formula:
336 x MPH x rear end ratio / tire dia.

:cheers:

Paul Borowski

David, I have this combo in my '68 convertible-was M-21 w/3.70s. The close ratio trans has the taller first gear(2.20, I believe)than the M-20 which has the 2.64 ratio?). The 3.08 IS NOT a good gear with the M-21. I do not at all race my cars, but with this combo I have to "slowly ease off" the clutch to get rolling. My 4spd '80 had a Warner wide ratio with a 3.07 rear end, BUT the Warner wide ratio had a 2.84 low gear, so it mated up great with the "tall highway" 3.07s. In you case, I would'nt go lower than a 3.36:1 with this your combo. My '75 with an L-82/automatic has this gear and I think the 3.36:1 gear is a very good cruising/performance gear IMHO. It's my favorite gear of all four of my cars. :yesnod:

Gordonm

:iagree: with all the answers. The 2.20 first gear with a 3.08rear is going to require a lot of clutch slip to get the car moving. It is going to be a dog off the line. You will be able to get to 60 mph in 1st gear easy. I think you might want to reconsider this. If you are going to keep the stock power the T5 5 speed might be a good trans for you. It takes a little work to get it in but the 5 speed will allow good acceleration with good highway manors.

redvetracr

If anyone is interested I have 3- 3.08 rear ends I would like to sell from a core to a bolt in and go...e-mail for more info. redvetracr@msn.com

Tom454

My 66 C2 327 has it's original 3.08's and it drives fine.

I can light the tires if I want to, and it cruises well.

I also have the original numbers matching "standard" muncie 4 speed that originally came in the car. Don't know the 1st gear ratio.

The 327 is a re-worked 2-bolt block out of a 67 Camaro with the 350 HP cam, plus forged crank.

Can't give you tach numbers because I have the engine out for a re-ring.
Never considered it slow off the line, and as a past President of a Corvette Club.. (Southern Tier Corvette Club, Binghamton NY), past member of Natural Glass Corvette Association of NJ, and Corvettes International in Raleigh, I put a lot of cruise miles on it (mostly in Binghamton).

Have owned it since 1974/5

I don't see any issues with the 3.08's since you specifically said you're not interested in racing anymore.

Tom



[Modified by Tom454, 8:37 AM 4/2/2004]

Glensgages

In a stick-shift application;
MPH x gear ratio divided-by .006 divided-by 1/2 tire height = RPM
75 x 3.08 div/by .006 div/by 13.5" = 2851 RPM.

I've never run 3.08s with a close-ratio 4-speed, but the equivilent ratio with an automatic off-the-line would be a 2.73:1 ring & pinion.
(2.20 x 3.08 = 6.776
6.776 divided-by 2.48 {1st gear in a THM400} = 2.73)

Even if you're not racing it, I'd think it's acceleration would leave something to be desired.

Just my $.02 worth...

Master Ho

Plug in your data here: http://www.2004r.com/speedoCalc/rpmcalc.shtml

(Oops! For auto tranny comparisons only!)


[Modified by Master Ho, 8:13 AM 4/2/2004]

Matt Gruber

here is an easy test to see if you will like it.
Drive around with the 370's, starting out EVERY TIME in 2nd gear.
i had 336 &2.20 in my 61 for years. i have no idea if you would like it, so try the above test.

Gage

cancel the appt!!! You will not be happy with the 3.08s and an M21. There is a reason they never left the factory with anything less than a 3.36 with an M21. The pleasure of the low rpm highway cruising will be offset by the frustrating drivability around town from stoplights...plus you will make all us vette drivers look bad because 17 yr olds in 93 mustangs and Ricers will be beating you

Bob Turner

:iagree:

Cali- Right now with the 3.70's and 2.20 first gear, you are running an 8.14 to one Final drive ratio. If you go with 3.08's, that will drop to an ugly 6.78 to one.

With my 74, I currently have 3.08's and an 8.53 Final Drive Ratio. I just purchased a set of Richmond 3.70's and will be running a 10.25 FDR. This BB has been a DOG with the 3.08's and I can't wait to get the lower gears in there to get the power to the ground much faster.......

I would suggest as others have to locate a 3.08 core, install it and see how you like/dislike it. Then, you would have the 3.70's to go back in later on if you choose to....

Bob :cool:

norvalwilhelm

I am running 308 and love them . BUT I have a Tremec with 3.27 first gear and I put out more then twice the torque you would.
That said I can see why you woulld not like the 3.70 on the open road and if high speed cross country running is what you want go for the 308's.
Gas mileage improves, motor runs cooler and engine wear is down.
7 to 1 out of the hole might be a little tough but you will learn how to do it and once you are rolling everything else will be fine.
I ran 4.56 then 4.11 for years and last summer was my first time with stock gears and I am much happier.

Matt Gruber

being cheap, i would just buy a pair of tall highway tires/wheels.
would change the effective ratio to 3.36 with the 3.70's. speedometer would be off ! unless u change the gear too

Cali,68,L-79

I'm going to drop by Pro-Gear and talk to the manager. Well see. Looking at the replies it looks like 336 may be the way to go. I do not want an absolute dog off the line but at the same time I do not want with 370's to top out at 95mph nor cruise speed (75mph) at nearly 4 grand.

Thanks for the replies.

The Dude

Dave, I have to second (or third or fourth) the suggestion that you will probably regret going to 3.08s with your transmission and engine combination. 3.55's may work better for you as a compromise.

Brown78

I run a 78 L-82 came with 3.70's. I live in south Florida, great roads flat and straight :auto: I couldn't run with C5's. at 70 mph. Changed the rear to 3.08 my RPM droped 600 revs. Before 60 mph was 3000 rpm's Now 70 mph is 3000 rpm's. To me, that is not a high RPM to run at. been running 30 to 3500 rpm's for the last 2 yrs. no problem. I'm 56 yrs old. I let the kids do the light to light. And I just REMINISCE about 69,70,71,72 and all the hole shots. I just cruise and bracket race once in awhile :cool: Life is good!!! :lol: :seeya Steve P.S. Same size tires w/ Borg-Warner T-10 :steering:

BBShark

Go for a 10:1 final drive ratio (FDR). You are in 1st gear a lot more than any other. 10:1 is the magic number and the OD will be sweet at 70MPH. Always listen to Norval, he straightened me out on this very subject.