Gear ratio help

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Work hard play always

Work hard play always

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I just picked up my new 71 Valiant with a 225 slant 6, auto. I believe I have the 7 1/4 rear end and it howls like a mother at 70.
Since its nothing special Im not gonna fix it just swap it out. I have sourced several on craigslist for cheap. Can anyone help me with what to look for to make sure I get one thats a direct swap, ie: gear ratio and if they changed during specific years. The cleanest one I am looking at came out of a 70 dart but I think it was a v8 car. I am also planning on taking the springs since one of mine sags an inch. This valiant is just a toy and not a speedy car.
 
Slanty acceleration sucks with 2.45s, so stay away from those,lol.
but they do pretty good with 3.23s.
Mostly you will find 2.73 to 3.23s
If going to a ratio other than what you have, you will need the matching speed-o gear.
 
First, don't pay too much. 7 1/4s are almost give away. I'd say $100 tops for an open diff; the sure-grips are another matter.

If you come across a newer one, pay attention to the bolt pattern because I'm not sure when they switched from SBP to BBP, but someone here can provide the details. If you do in fact have SBP and you find a BBP rear for a good price, I think the axles are pretty easy to remove/replace on those.

And the later V8 cars may have a 7 1/4 as well. I have a '73 Swinger that's a 318 2bbl automatic and it's 7 1/4 2.76 from the factory. I'll be going to an 8 1/4 when I do the 4bbl swap, but for what you're doing there's really no point to anything bigger. But as AJ pointed out, finding one with a better gear ratio (higher numerically, assuming yous is a 2.xx) could be a nice upgrade.
 
Good idea upgrading the 7 1/4 to an 8 1/4 if it’s already howling. Not much point fixing a 7 1/4 unless your on a tight budget. I’d consider buying new springs as any used springs are just that; all are 40+ years old and will need to be re-arched sooner or later.

As others have suggested, consider a 3.23 ratio. I did that exact swap several years ago and am happy with it and was even able to sell my 7 1/4 for $50! Went from a 7 1/4 open w. 2.76 to 8 1/4 sure grip w. 3.23 and bought new HD springs. The 8 1/4 for A bodies started being installed in ‘73. You will also need to shorten your drive shaft ~1.5” when you install the 8 1/4. Other than that, it’s a direct swap.
 
Work hard play always said:
How do I find the gear ratio?

Not sure what I have and is there a way to find it on the donor rear end? (removed from the car?)
Click to expand...
I NEVER bother with tags, I just "turn it over." This gives you a chance to see if it's limited slip, which in a 7 1/4 "ain't gonna happen."

Remember that turning just one wheel you have to turn it TWO turns to get the shaft ratio. So if the opposite wheel is stopped, and you turn the jacked wheel TWO turns, the shaft will turn whatever the ratio is, IE about 3 1/4 turns for 3.23, not quite 2 1/2 for 2:45, whatever it is.
 
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