I'm restoring my differential, and thinking about going back to original 3.70 as per codes on the pumpkin. Currently the car has 3.08 that were switched before I got the car and with the close ratio M21 and stock L-68 427 with 27" tires it pulls and drives just fine. It cruises nicely but could use a little less gear on take off. I was thinking a 3.36 would be a good gear for this car to give me a nice take-off in first gear and a good cruising RPM.

Checking out some calculators on the web, I get the following:

3.08: 2108 RPM @ 55mph
3.36: 2300 RPM @ 55mph

and then...

3.70: 2532 RPM @ 55mph

and then there is...

3.55: 2430 RPM @ 55mph (for posterity)

At the 3.70 ratio it seems like a lot of RPM to do 55... but I had always thought that that was a more mid level gear ratio along with 3.55, and the 4.11 and 4.56 and lower would be considered really low. Problem is my OCD keeps telling me that the 3.70 gear would make this car more "correct" for all that work since I'm trying to put the car back as original as possible... so I keep wanting to go that route. This is a lot of work so I only want to do it once.

So question is, guys with similar cars (L-36 or L-68 427, close ratio trans, stock tires); what gear are you running and how is it for a a nice cruising car? And what would you change if you could? Anyone with a similar car have a recommendation? Seems like a lot of L-36/4spd cars were 3.36 IIRC.

I honestly don't know how the guys with 4.11 and 4.56 gears can even stand to drive the cars... they must be screaming all the time just to go down the road at the speed limit... That would drive me crazy...


thanks!

 

My car has no similarities to your car but I thought I would weigh in on your last statement. The higher (numerical) gear ratios provide much improved acceleration capabilities. If acceleration is important to someone, they accept the downside of high rpms at cruising speeds. BTW - I'm always glad to see someone else on the forum who realizes gear ratios are written as 3.08 rather than 3:08. I guess they are confused by the 3.08:1 notation.

 

A street car is great with a 10:1 in the drive train.
With your m21 and 3.08 you have 6.776:1

Today if you drive 70mph you are @ 2683rpm

If you go to 3.55 you will get 7.78:1 and 3093rpm (70mph)
3.70 will be 8.14:1 and 3223rpm (70 mph)
4.10 will be 9.02:1 and 3571rpm (70 mph)

 

the answer is easy, 3.70

 

I am dealing with the same issue. I have a 68 with the L79 327 , 4 speed and it has 3.08 gears really not much fun to drive takes off like a dog. I am debating between 3.55 and 3.70 as I do not do that much freeway driving I am leaning toward the 3.70. In your case depends on your use I would lean toward the original 3.70.

 

I actually think this sums it up... how do I want to drive the car. Do I want to be a stop light drag racer and do ridiculous burnouts? Or do I want cruise around and enjoy the scenery? Maybe in my 20's I'd have preferred the lower gears for more street heroism, but now that I'm in my 40's I think I want to just cruise around and enjoy this great car.

Going back to the 3.70s for the sake of originality is only going to help me on the day I sell it... maybe... and then maybe not. Even if I NCRS'd the car they would never know it had 3.36's instead of 3.70s. For now I think the answer is to set the car up to drive the way I want it to drive.

Thanks for the perspective, this was very helpful.

 

Robert, I think the difference between my car and yours is the big block vs. the small block. My car has gobs of low end torque so it pulls the 3.08's around without a struggle. The only thing really is you can't go slow in it in first gear, like parade slow or up my drive way slow. Your 327 may have to work a lot harder out of the hole than my 427, so it would work a lot better with the lower gears like you said. I probably will now have a very nice set of GM 3.70 gears to sell, shoot me PM if you are interested.

 

Admitted I got a bit confused,

I 427 is so sweet, will rev faster than a 454 but still has great low end torque unless rectangle ports,

"Most" well built as in non smogger big blocks are happy to smoke most any street tire with any gear in the 3's,
I could likely run a gear in the 2's and still smoke tires with my 454.

So, do you want more low end dig as in from a stop or less low end dig?

 

I have 3.36 in my '70 and it is a nice highway cruiser taching 3000 rpm at 70 mph. This has allowed me to drive it to Bowling Green and other Corvette events several states away. I would have never done this if it had a 3.70 gear....something to consider

 

Manual trans like a little more gear than an auto, and a close ratio M21 doesn't help. 3.36 and 3.55 are your best choices - depending on how much cam and how much highway driving there is.

$.02

 

3.36 is the best "all around" gear. You can actually keep up with traffic on the highways (70 mph+) and still have good acceleration. Auto or stick, it's a good gear.

 

Good luck with everything.

What I keyed in on is that you have a close ratio tranny.

My 70 LS-5 has its stock 454 with original wide ratio trans with original 3:36 rear. I absolutely love this car and the way it shifts.

That being said I have a little concern about your tranny being a CR. Based on this I would strongly consider the 3:55. This is based on your CR trans and having to slip a bit more on take off. Clearly 4 th gear is 1:1 so that is not of concern. If you don't mind riding the clutch then I guess the CR trans is not a big deal. Read a lot of posts on here and the general consensus is the 3:55 is about as low as you want to go for a CR.

Now. My personal opinion as to why I like my set up is the difference between the torque of a big block verses the SBC.

In my 63 I have a 3:36 and WR trans and the experience is very different.

I'm just thinking that your 427 will turn up faster so you might like the 3:55 better. And with its torque you will have a little more fun factor.

I feel for you as these are hard decision and are tough to swallow if you are not happy with the end result.

 

I see that you've already decided. The P.O. put the 3.08s in strictly for highway driving since the big block has more power to handle them.

If you REALLY want to know "what it's like for your engine", remove the mufflers and drive with the 3.36 gears...that sound you hear at 60 mph will be what the engine is "up to" with 3.70+ gears.

Reasonable gearing for each driving "style":
Highway = 3.08 to 3.36
Mixed = 3.43
Boulevard= 3.55 to 4.11

 

Ps. Don't forget to change plastis speedo drive gear to correct one for your rear gear choice.

 

Some very helpful advice here, thanks! Right now I'm between 3.36 and 3.55, and leaning towards 3.36. I'm also considering if I'll want to add a taller tire later.

 

Especially if "lower ets" are not your biggest concern.

 

Ohhhhhhhhhhhhhhhhhhh......removing the mufflers "got you to thinking"!

(Just picking!)

If you go 3.55.......and later you decide to do more highway driving (or decide that 3.55 is a bit too much)....taller tires would be the next step.

Use a Tire/Gear chart to buy the correct diameter tire for the highway RPM that suits you best.

 

The 3.55 gears would be better for cruising. I had them in my L-79 four speed, but changed to 3.70's because I hated to have to slip the clutch to start from a dead stop every time. With the 3.70's, you just let out the clutch and go. Lou.

 

Hi Moon, my 2 cents is your on the right track with 3.36 or 3.55. although Lou's 68 s/b with 3.70;s is a great car to drive around in. I am going to run 3.55 and see what happens. I just looked at the pictures of your ride again and it is one beautiful chrome bumper car, and I love the story you told me when I met you. Good Luck.
R

 

Roger! Thanks, your 2 cents is always welcome in my book. The car is coming apart probably tonight for the differential swap and I'll be ordering the 3.36's from Tom's as soon as my credit card cycles. Plan is to have it all back together by the end of February.

Going to check out your restoration thread now, I never did see how you made out with the birdcage. Hope all went well...