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Originally Posted by Tom Kirkham
3.31 and a CR Top Loader are only good for big long tracks. And MMP is not long enough. For street use this combination is very hard on the clutch. I know, because that is what is in my car and as soon as I have some spare time I am going to change either the trans and make it more of a street car, or change the diff gearing and make it more of a race car. Basically, the CR top loader transmissions are best for racing and WR are best for the street. Note that the TKO600 CR gear ratios are almost the same as the top loader WR box with the addition of an OD gear. All of our customers who have WR TKO600 have not been very happy with the gear split from 4 to 5.
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I think the 3.31 rear doesn't really fit either the CR toploader, or the WR toploader, or shgould I say take advantage of the gear ratio's of the tranny. It's not steep enough for the CR toploader, like you eluded to, and with the 2.78 1st of the WR toploader, it's maybe too steep, and really doesn't give that much advantage in street cruising. I think the 3.54 is fine for a CR toploader, and for the limited highway cruising, it's fine for that as well. The 3.07 rear takes advantage of the WR 2.78 1st, and also allows you to cruise with the 3.07's on the highway.
I would guess the perfect rear gears to use for a CR toploader would be 3.54's or 3.77's, and with a WR toploader, 3.07's, and 2.88's.
On paper, I could tell that the TKO600 with the 0.68 OD would be way too much of a jump from 4th.
My stang runs a CR toploader with factory 3.91 rears, and it's fine for highway driving. My Olds has a M-21, with 2.20 1st, and I run 3.36 rears, and its fine as well, starts out fine and cruises fine
You can make about any combination work for the street, the question is what's optimal for what you want to do? Everything is a compromise, unless you get a custom CR T-56, or a Richmond 5-speed. Then you have the true close ratio gears, and can gear the rear for cruising as well.