What Rear gear to use ?

[Mike Cobine]

Try running A MPH Calculator from RPM and Gearing

This will give you a chance to play around and see the impact. Of course, you can try many different values, then compare them for actual feel by seeing which is close to taking off in second or taking off in third.

While Joe is partially right about the best choice is the one made by Chevrolet, you have to remember that was based on many different needs of the time:

1. Corvette was a daily driver, high performance car.
2. It was driven in a high performance manner.
3. It was not a show piece or an investment.
4. Gas was about $.25 a gallon for premium (Ethyl, whoever she was.)
5. Getting 8 mpg was zero consideration.
6. No one cared if you wore the engine out in two years of driving like this as the engine would probably be blown up drag racing or road racing long before.
7. The need for a close ratio 4 speed was based on road racing and drag racing, so that the engine could be kept in the narrow power range on the top of the horsepower curve. Do you typically drive in the 5000-6500 RPM range and stay there?
8. The 3.70 standard and 4.11 performance was necessary due to the close ratio to get any decent take off performance.

My opinion? You would be better off pulling both the 4.11 and the CR 4 speed out and replacing with a wide ratio 4 speed. If you still want some wild take-off from the traffic lights, run a 3.55. If you want to cruise 2000-mile trips, go with the 3.08, although you may have some stumble on take off, especially on hills.

3.36 would be a good compromise and was the Chevrolet standard with a wide ratio four speed.




A MPH Calculator from RPM and Gearing

[Joe R.]

Re: Try running A MPH Calculator from RPM and Gear

I have to agree with Mike Cobine that the best "all around" combination would use the wide ratio Muncie instead of the close ratio. Close ratios were popular for racing applications where the engine was typically kept in the upper end of the RPM range, but the wide ratio is much better for normal street driving.

One way to look at this is to consider just first gear. The CR first gear is 2.20, while the WR first gear is something like 2.52. So, the effective first gear with a CR plus 4.11 rear is (2.2)x(4.11)= 9.04. That's about the same as a WR with a 3.55 rear, (2.52)x(3.55)= 8.95.

In other words, acceleration in first gear should feel identical with either of these two combinations, while RPM in 4th gear will be 14% lower with the WR and 3.55.

I think it is pretty straightforward to swap the WR gear set into a CR transmission. However, the cost will probably be high if you buy all new gears.

By the way, if you have your rear end rebuilt by Bairs, you can get a new ring and pinion with any of the standard ratios, but it will cost few hundred dollars more than if they reuse your existing ring and pinion.

[Mike Cobine]

Actually, change the entire unit

That is what I usually do. Change the entire four speed, not just the gear set. Then to be correct and original, you change the transmission back in an hour instead of having to rebuild a transmission.

Same with the rear end.

Buy an entire rear, swap it out. If you watch around, you can get a good deal on the whole piece, although you won't get it at most of the big name vendors who want to sell you a refurbished rear end for $1200 or a all-new-parts rear for $2000. I've never spent over $350 on one and that usually had suspension with them.

[`Michael Southard]

Re: Try running A MPH Calculator from RPM and Gear

WHEW!!!
Everybody had me pretty much convinced that my 3.36 sitting on the bench was worthless. (Well, I guess it is on the bench!)

But I'll put it back in, in a month or so as I continue my resto. Thought about changing it, til your post.
Mike

[William M.]

Re: What Rear gear to use ?

I've always felt a 3.36 was best & a 2.54 box; just got a '66 390 with that gear but 2.20 & its a little harder to deal with from a stop.