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I just purchased a 55 bel air, it has a 350 with a M-21 4 speed. The ford 9" rear end is geared far too tall with a 3.08:1 I am looking for a good street and half decent highway gear, was thinking 3.70:1 would be a good compromise? any suggestions?
 

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X3 on the 3.55 gearset ; if you want a red-light bandit go to a Big block . The 3.73 will be a screamer on the highway with anything less than a 28" tall tire . and you boys have some LONG straight highways . Glad you kept the old M21 ; they've got great first gears. Be safe...Bob.
 

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I've got the same 350 m21 setup but with 3.25 gears and 27" tires. What little I have driven seems the gearing is pretty good for cruising. With the kind of driving I do I'm not sure I would want to go any lower on the gearing. Got a buddy with a 383 turbo 400 combo that went from a 3.75 to a 3.25 and loves it. The 3.50 would probably work too just more RPM and fuel than I want.
 

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I just purchased a 55 bel air, it has a 350 with a M-21 4 speed. The ford 9" rear end is geared far too tall with a 3.08:1 I am looking for a good street and half decent highway gear, was thinking 3.70:1 would be a good compromise? any suggestions?
Like previous posts a 3:55 would be a all around gear. My 57 does 3 grand at about 60 mph. I like the car squirrely on occasion.
 

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Since you are guessing on current ratio I suggest you poll the jug and confirm what you have now before buying anything, How do I know you are guessing because 3.08 is not a 9 ibch ratio.
 

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Few of the other ratios mentioned are available for a 9".

These are available (and more):
3.00, 3.25, 3.50, 3.70, 3.89, 4.11.

What you need to decide is how much rpm you want at cruise.
 

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Thanks for all the input, I just pulled the info on the car from when I bought it and it has a 3.08 "Hogs head" rear end... Gotta do some homework...
Posi or open? You could probably run an ad in the classified forum for a trade.
 

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Nothing wrong with a 3.08 for a combo highway and town car.

This was a common rear gear with the wide Muncie.
 

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USA1 is correct. 3.08 is not a 9" ratio.
A 3.55 9" ratio is not standard either, although you can get it in a 9" 3.50 is more common.

3.50 would be better if you think your gears are too tall.

I like 3.08 gears with a high torque engine.

Don
 

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Lots of different terminology gets bastardized over the years ; some call them pots , some pigs , some carriers , some hogs' heads , some just diffs . Depending on the circle you travel in and where you were brought up , the terms change , but we all know what the OP means . He'd probably do fine with the 3.0 gearset and the M21 , if he had more motor . Without an over-drive , it's hard to build an all-round street machine that won't break the bank on the highway. Be safe...Bob.
 
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