Gear ratio for 7 1/4

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MassDart

MassDart

Lifelong Dart fan
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So here's a question. Currently my 1967 Dart has a 7 1/4 with presumably 2.93 or worse gearing. I bought the car with American Drag 15" rims, so my guess is that an already lousey gear ratio is made worse by the tire size (which I actually like on the car). My question is, that ideally, I would like to have 3.73's or so, but I haven't seen any ring and pinion sets for that rear diff. available. What I have seen, is 4.10's. With my increased tire size, would 4.10's be reasonable end result? The car also is an automatic with 904 and 273 motor. I'm planning on camming and carbing the car in the near future. I'd like the car to be snappier off the line, and I'm not really planning on using the car on highways much or at all. The car only has 36000 original miles.

On another note, If I decide instead to try to find an 8 1/4, what is the easiest model to look for to swap? If I've read correctly, try to grab one out of 91ish Dakota (?) I think. Is that an easy swap, or do I have to reinstall the leaf mounts etc?

Thanks for any advice.
 
What is the tire profile ie.. 235/70/15?
 
If you are thinking of swapping in an 8.25 rear find one out of a '73-'76 A body, it will bolt in using the 8.25 u bolts and shock plates. But be aware that will change your wheel bolt pattern to 5x4.5" from your current 5x4" bolt circle , so your wheels will no longer fit. The driveshaft length will need to change by about an inch or so.
Dakota rear ends were wider than A bodies, and used a six lug axle after 1990.
Easiest upgrade is a 8.75 axle out of any '65-'72 A body, but they are pricey.
If you aren't doing any highway driving, 4.10s in your 7.25 wouldn't be ridiculous. I'm just surprised you could find them for the 7.25, and that would probably still leave you with an open rear, since most likely you've got a non-suregrip. And suregrip units for the old 7.25s are next to impossible to find. It's getting difficult to find ANY service/repair parts for the older 7.25s.
 
I would not put 5 cents into regearing or doing anything to a 7 1/4 'specially with a v8 and you want to pump it up some

Dakota is a poor choice as you'll have to cut off the spring pads and start over, and the bolt pattern is completely different. You'll likely have to change the axles, I doubt you could re-drill the 6 bolt pattern. The early Dakotas may have? been 5 bolt but you still need to rework the perches and I don't know about width. I bet they are wider

As said you could look for a later 8 1/4 and figure on maybe later changing the front over to 4.5 bolt circle
 
if you plan on pumping up your 273 and put 410 in the rear the 7 1/4 will not last long. They are weak and not really meant to be behind anything with performance. I was able to find a set of 3.21 rear gears about 10 years ago available from online rear diff companies. I would only trust a 7 1/4 behind a slant 6. Even though the cubes from a 225 to a 273 is minimal, the torque is higher and that is what will destroy the 7 1/4. I have blown out quite a few 7 1/4s even behind lightly modified 273s

8 3/4 is the best option but pricy 8 1/4 is a good choice too but as with everything getting very hard to find.
 
Okay. Thanks all for the tips. I think I'll just save my time and money and look for an 8 3/4
 
Professor Fate said:
If you are thinking of swapping in an 8.25 rear find one out of a '73-'76 A body, it will bolt in using the 8.25 u bolts and shock plates. But be aware that will change your wheel bolt pattern to 5x4.5" from your current 5x4" bolt circle , so your wheels will no longer fit. The driveshaft length will need to change by about an inch or so.
Dakota rear ends were wider than A bodies, and used a six lug axle after 1990.
Easiest upgrade is a 8.75 axle out of any '65-'72 A body, but they are pricey.
If you aren't doing any highway driving, 4.10s in your 7.25 wouldn't be ridiculous. I'm just surprised you could find them for the 7.25, and that would probably still leave you with an open rear, since most likely you've got a non-suregrip. And suregrip units for the old 7.25s are next to impossible to find. It's getting difficult to find ANY service/repair parts for the older 7.25s.
Click to expand...
Do all 73-76 A-Bodies have the 8.25? Even the S6 cars?
 
The King said:
Do all 73-76 A-Bodies have the 8.25? Even the S6 cars?
Click to expand...
No, the 7.25 was still in use in '73 and up, just with a big bolt pattern when front disc brakes were ordered- mostly used in /6 and light duty 318 cars (some small bolt patterns were still being installed in '73, but those were only in slant 6 cars with front drum brakes- yes, it can be confusing). The 8.25 was the replacement for the 8.75 in '73 and later cars, used in 318, 340 and 360 applications, though some may have found their way into slant cars for HD applications. Never hurts to check.
 
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