Need advice for diff for 65 Dart factory hi-po 273.

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Stumpy

Stumpy

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The rear end is whining on my just bought 1965 Dart with the factory hi-po 10.5:1 273 4bbl 4-sp set up with a 7.25 rear end. The tires are 205/70 R14s.

I'd like a 8.75 but it will be $$$ and time. Maybe some day, but for now I'm thinking to just keep the stock diff. It lasted 50+ years and I won't be beating on the car and way cheaper. Am I just wrong about this. Did a quick search and A body 8.75 look to be as rare as hen's teeth and I really want to return this car back to as original as I can with some modern upgrades.

The question is what gears should I order for it? The car won't be raced, and I'd like to be able to drive on freeway without winding it up with nice mid-range acceleration for merging or passing.

What was the most usual gear ratio for these out of the factory?
 
Check to see if you have a sure grip, both wheels turn the same direction. Could just be axle bearings. Usual gear was 3.23 on HP cars.
 
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3:23 is a good gear. Plenty of pep in my 66 my Hi-po car and almost 20 mpg/ hwy.
 
Put an 8 3/4 posi n it with about a 3:55 ratio and I think you can flip gears in that trans to make fourth into third and third into overdrive. It would be an awesome driver and it's a good investment.
 
66fs said:
Check to see if you have a sure grip, both wheels turn the same direction. Could just be axle bearings. Usual gear was 3.23 on HP cars.
Click to expand...
 
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If you can find an 8.25 diff from a 73 up A body it would be a good choice. More than adequate for a 273. It would most likely
be BBP from a V-8 car.
 
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If the car already had these upgrades, the previous owner could have sold it for a lot lot more money. Probably $3k more. If he would have sold it at all..it would be a pretty great car; as it is, it's just a local cruiser ready to break down.
 
CudaFactHackJob said:
Put an 8 3/4 posi n it with about a 3:55 ratio and I think you can flip gears in that trans to make fourth into third and third into overdrive. It would be an awesome driver and it's a good investment.
Click to expand...
No you can't flip gears.
 
The closest I can find so far for a 7.25 is 3.55. If someone has a lead on where I can get a set of 3.23s please let me know.
 
Stumpy said:
The rear end is whining on my just bought 1965 Dart with the factory hi-po 10.5:1 273 4bbl 4-sp set up with a 7.25 rear end. The tires are 205/70 R14s.

I'd like a 8.75 but it will be $$$ and time. Maybe some day, but for now I'm thinking to just keep the stock diff. It lasted 50+ years and I won't be beating on the car and way cheaper. Am I just wrong about this. Did a quick search and A body 8.75 look to be as rare as hen's teeth and I really want to return this car back to as original as I can with some modern upgrades.

The question is what gears should I order for it? The car won't be raced, and I'd like to be able to drive on freeway without winding it up with nice mid-range acceleration for merging or passing.

What was the most usual gear ratio for these out of the factory?
Click to expand...
I think you better look things over and count the turns. It sounds like you aren't sure what gear is in it. No reason to order a 3.55 if it has a 3.23 in there. If the rear end is good, working , and not noisy I would leave it alone until you can find something better. 7 1/4's can't take much abuse so don't bet on it. They have to be set up in the car and are a turd to do even for the experienced mechanics.
 
toolmanmike said:
3:23 is a good gear. Plenty of pep in my 66 my Hi-po car and almost 20 mpg/ hwy.
Click to expand...

I don't get it. I'm running 8:25 with 3:21 gears. Pulled off my rpm air-gap and put performer intake on earlier this summer with AVS-2 650 edelbrock. Mild 318 250hp 904 tranny. 25" tires in back. Anyway, was getting between 12-13mpg maybe 13.5 on hwy. After switching intakes maybe picked up 1/2 mpg. We do alot of cruising and would love to improve mpg somehow. I keep thinking since changing intakes I could get by with a 500 eddy. Anyway, saw your post and thought "how does he do that?"
 
gtxdude said:
I don't get it. I'm running 8:25 with 3:21 gears. Pulled off my rpm air-gap and put performer intake on earlier this summer with AVS-2 650 edelbrock. Mild 318 250hp 904 tranny. 25" tires in back. Anyway, was getting between 12-13mpg maybe 13.5 on hwy. After switching intakes maybe picked up 1/2 mpg. We do alot of cruising and would love to improve mpg somehow. I keep thinking since changing intakes I could get by with a 500 eddy. Anyway, saw your post and thought "how does he do that?"
Click to expand...
9.6/1 HP 273 with the stock open plenum intake and a 600 Edelbrock. It has a Isky E4 cam still running the solid lifters. I also run the original dual point distributor with 8 ° initial and 32° total. Stock exhaust manifolds with Accurate LTD Hi-po single exhaust. I get 18+ easy and have gotten 19 on a 2 hour drive @55 mph. Oh, original 4 speed. (non overdrive)
 
gtxdude said:
I don't get it. I'm running 8:25 with 3:21 gears. Pulled off my rpm air-gap and put performer intake on earlier this summer with AVS-2 650 edelbrock. Mild 318 250hp 904 tranny. 25" tires in back. Anyway, was getting between 12-13mpg maybe 13.5 on hwy. After switching intakes maybe picked up 1/2 mpg. We do alot of cruising and would love to improve mpg somehow. I keep thinking since changing intakes I could get by with a 500 eddy. Anyway, saw your post and thought "how does he do that?"
Click to expand...
There are so many ways to lose fuel economy , it ain't funny.
Economy starts with a lightweight car, a small highly efficient engine, very low cruise rpm, and ends with major tuning session.
But if I had a 318, it would be knocking on 24mpgUSg, Cuz it wouldn't be stock.
It would have alloy heads, a solid-lifter fast rate of lift cam, and a Dcr pushing 9/1 The cylinder pressure would be pushing 190. It would have a 4bbl with triple booster primaries, headers, and a free-flowing exhaust with dual full-length large diameter primary pipes, and huge triplepass muffs.It would have a close-ratio 5-speed manual trans and cruise at say 65=1800 rpm. I would stick it in a lightweight chassis of no more than 3400 car and driver.... cuz I already have one lol.
I bet I can tune that hi-compression teener to do better than 24.I mean I have a 367, that in one iteration easily surpassed 24, so a 318 should be a breeze.

But yer not likely ever gonna get that out of a stock low-compression LA-smoggerteen with an A904, and I don't care what gears it runs, or how light a factory chassis you stuff it into. Well, maybe you can; but I sure couldn't,lol.

The point is this. if your 318 is only getting 13.5mpgs, IMO, there is either something you're not telling us, or your combo is in serious trouble.,
 
3.23 is a good ratio, 3.55 is even better! 273's like to rap. RPM's dont always dictate MPG.
 
CudaFactHackJob said:
Put an 8 3/4 posi n it with about a 3:55 ratio and I think you can flip gears in that trans to make fourth into third and third into overdrive. It would be an awesome driver and it's a good investment.
Click to expand...
Posi SG.jpg
 
I agree with 66FS. Axle bearing are most common source of noise and vibration. Check those first.
 
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