What Rear gear to use ?

[Tom S.]

I need some advice as to what gear to use. I have a 69 l-46 with a m21 tran. The gear in it is original and it is a 4:11. The problem is I have it out and there is quite a bit of end play on the axles.I have not been happy with the ratio as it turns to high on the road.3500 rpm at 65-70 mph.It would be a good time to gear it up, what would be the best ratio? I am thinking a 3:70. The 1st gear in the M21 is 2:22-1 so I don't want to go to high.Thanks in advance for the help.

Tom Stanton #41491

[Duke W.]

Re: What Rear gear to use ?

My SWC has a 340 HP engine, CR trans, and 3.08 axle. First is good for 75 MPH at 6500. The first 40 is not too impressive, but it sure flys (literally) on the top end, and 80 MPH is only 3100 revs in fourth.

Your revs at any speed in top gear can be computed by multipling revs by the new ratio divided by the old ratio.

The rear gear on a CR 4-speed is always compromise. Mine is like a five speed with no first and yours is like a five speed with no fifth. It depends on your driving conditions and what's important to you. The rear gear is only important to acceleration below about 40 MPH. For example my first gear is about the same overall as your second.

If you go with a lower numerical gear you will have to get a new case as there were two and the split point was 3.70/4.11, but you probably need a whole new Positraction anyway. New Eaton cases are now shipping after being tough to find for the past several months.

Back when these cars were new most guy wanted short gears for quick accleration. Now most of the same guys want tall gears for easy highway cruising.

Duke

[William C.]

Re: What Rear gear to use ?

I use a 3.36 in my '62 fuel car with a CR 4 speed. Like Duke says, a little sluggish off the line, but you really don't notice it in general driving.

[Mike M.]

Re: What Rear gear to use ?

duke and bill: you guys are aging. not me. can't beat 4.88:1 with the ZF6.first gear a stump puller and in 6th gear at 2200, its running about 75 and getting 22 mpg. everybody ought to have a "farm use" vette. mike

[Michael H.]

Re: What Rear gear to use ?

Mike,

Sounds like you have the best of both worlds with that range of ratios. However, for those of us stuck in the 60's with old fashioned 4-speeds, I'd have to go with a 3.70 for all around perf/cruise. (maybe 4.11 for a FI small block) I guess diff ratios are always going to be debateable and that's probably why they offered so many different ones. My first new Corvette was a 64 365 HP with a 2.20 and 3.08 diff. I hated it but could never afford to change it.

[Wayne P.]

Re: What Rear gear to use ?

I always go for a 3.55:1 for a good all around gear. Reasonable take-off and reasonable rpm on the highway.

[Gary S.]

Re: What Rear gear to use ?

My 72 LT1 had its original 4:11 when I got it. The first 5000 miles were great, then I tired of the high rpms. Now, with a fresh rebuild on the diff and 3:36 gears it is much more fun to drive. No doubt that off the line the 4:11's were quite impressive. I bet I could substitute for a pace maker with those gears. Oth, driving to and from events and on road trips is now a pleasure. If you don't want to lose the true excitement go with 3:55's or 3:70's.
Gary

[Mark #28455]

3.55 w/ wide ratio muncie *NM*

[Ed Jennings]

Re: What Rear gear to use ?

My 75 has a slightly "built" L48, with wide ratio BW, and it gets around just fine with 3.36's. My 67 L79 has a 3.70, and I wish it had a 3.36 or even a 3.08. The 62 I sold recently had a 3.55, and it was a pretty good all around combination. The car was a fuelie and had come originally with a 4.11. I am thankful the PO had changed it, as it saved me the trouble. If I had needed to change the rear for some reason, I would have probably gone to a 3.36, but I sure wouldn't go to the trouble and expense just to change the ratio from 3.55. If you just drop down to a 3.70, you may wish you had gone another notch. If you go to a 3.55 or 3.36, you will be able to notice the difference.

[Tom S.]

Re: What Rear gear to use ?

It looks like most all prefer the 3:55. My only question now is will it be to tall with the 2;22 first gear?I would like to get as high as I can without being hard on the clutch.Thanks for all the response.

Tom

[Ed Jennings]

Re: What Rear gear to use ?

That 62 FI car I had with the 3.55 had a Comp Cams 280H in it, which is a real agressive cam (ie not a lot of low end torque, relatively speaking). I had absolutely no problem starting off in 2nd gear on level grouns without slipping the clutch excessively. Unless you are intent on smoking the tires on a regular basis, I think you'd be real happy with the 3.55, or even a 3.36, depending on your driving habits.

[Tom S.]

Re: What Rear gear to use ?

Ed
I don't care to smoke the tires.I just want a good all around gear that I can take on road trips of up to 2-3 hours without the engine screaming.From the input I have so far I think maybe the 3:55 will be the best for me it should still have pretty good pick up,not like the 4:11s but I don't plan on racing anytime soon.Thanks for your help.

Tom

[bill stephenson]

Re: What Rear gear to use ?

Tom,

--------I also vote for the 3:55. Our 66 427/390 just seems to love that ratio. Every car has that cruising sweet spot and our 66 will sit at 70mph all day long..........Bill S

[Tom S.]

Re: What Rear gear to use ?

Thanks for all the input. 3:55 it is. Now all I have to do is find one. Already checked the driveline, none in there.Shouldn't be to hard though I geuss. Does anyone know what the most popular ratio was? What was the most popular tranny? Thanks again for all the help.

Tom

[Joe L.]

Re: What Rear gear to use ?

Tom-----

For a stock drivetrain (or, a more-or-less stock drivetrain), selecting the best ALL AROUND rear end ratio to use is VERY easy. Someone else has already devoted a lot of effort to it. That "someone" is GM. Just check out the STANDARD ratio that came with your particular engine and transmission. In your case (1969 L-46 with M-21) that ratio is 3.70:1. That's the one that I'd recommend for ALL AROUND use.