1961 rear axle ratios

[harland]

Good morning, wonder if I could get some help understanding why Chevrolet used, per the owner's manual, a 3.36 :1 ratio for the 3 speed manual, but a 3.70:1 ratio for the 4 speed . I read that the powerglide had a 3.55:1 ratio and I know that usually automatics have taller rear ends , though I confess I don't know the mathematics involved. Your patient education is appreciated. Harland

 

[Tom Bryant]

I suspect that the ratios were intended to give the Corvette a little better off the line performance. In that time period it seems like in full size passenger cars if you got a V8 with Powerglide you got a 3.08 as standard. The low gear ratio in a Powerglide is very much a compromise since high is 1:1. 6 cylinder Powerglides had a lower low gear than V8s also and lots of guys swapped them for better performance in drag cars. I believe that the 3.55 was used strictly to give the Corvette better off the line performance with the 2 speed trans.

I don't have the info in front of me right now but I believe that the 3 speed also had a lower 1st gear than the 4 speed so it used a taller rear axle. I think that before 1962 all 4 speeds were 2.20 1st gear ratios and in 1962 there was a lower 2.54 available depending on engine (250 or 300) and option box checked. The lower 2.54 would be easier to get rolling with the 3.36 than the 2.20. Also the 3 speed car wasn't considered to be as high a performance car as the 4 speed. Full size cars in the mid to late 50s usually got the 3:70 with a V8 or 6.

My 1959 has a non-original 3.36 installed and with the 270 horse engine it doesn't like to roll away from a dead stop without some clutch slipping. I had a 3.70 in it that died and it was a breeze to get rolling.

Tom

 

[wallyknoch]

Hi Tom,

Your probably correct on all your talking points but nearly anything under the sun was available from a 3.36-4.56. I would guess that dealer ordered cars for inventory purposes would have followed the advertised recommendations or general purpose gears as opposed to special orders as I did on all my Vettes that were either 4.11 or 4.56.

 

[harland]

rear axle ratios

Thanks for the info. As I live in hilly country in CO, I won't be tempted to try for any taller rear ends. I think the big issue with them related to top end speed and lower RPMs. I don't feel comfortable driving a 1961 Corvette very fast-65, maybe 70, but that's it. Why chance injuring it on the super highways with so many bad drivers all around you. I didn't see the auction, but a fellow tells me a white 1961 Corvette with an accessory hardtop, small engine version, with a non matching number status, went for over $100K and that is something! That is amazing. Regards, Harland

 

[JohnZ]

Because through 1961, the 4-speed was only available as a close-ratio transmission, with a 2.20:1 1st gear; that ratio doesn't work very well on the street with anything less (lower numerically) than a 3.70 axle.