Best Gearing for 8 3/4 Behind 1970 Duster 318

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maldridgeks

maldridgeks

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Hello,

Recently purchased a sharp driver quality 1970 plymouth duster. Car has 318 with Edelbrock Intake and Holley 4 Barrel Carb paired with 3 speed 904 trans and 8 3/4 rear end with non-posi 2.76 gears. Rear tires are 255/60/15 and front tires are 215/70/15.

I am planning to convert the rear diff to sure-grip and upgrade the gearing. I am considering 3.55 or 3.73 and plan to mostly drive car in town but will occasionally want to drive short distances (30-45 miles) and would like to be at highway speeds up to 60mph.

Curious to hear if anyone has built a similar car and has experience with a build like this?

Thank you in advance.
 
plug your info in here to see what you'd be turning on the highway and if you can live with that
spicerparts.com

Spicer Transmission Ratio RPM Calculator

Use the Spicer Transmission Ratio RPM Calculator to determine your engine’s RPM based on the transmission gear ratio, tire height, MPH, and ring and pinion gear ratio.
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for what your intended use is, you'll be fine with 3.55's or the 3.73's

worst case is that you have some abysmally tiny cam in there and it just runs out of breath real quick
 
My Swinger had 2.76's and I switched to a 3:23 (with a 255-60/15 tire) The 3.23 is a good gear but I may get out on the highway driving 4 hours to a car show. I can see where a 3:55 or 3:73 gear would be a lot more fun cruising around town. I don't do burnouts or race so it really wouldn't be needed.
 
3.23 is a good compromise gear but a little weak for what your stated use would be. Imo, 3.73 is a little over the top for freeway cruising. That leaves 3.55, which would be my choice.
Coincidentally, I drove a 55 Chevy for 100,000 miles..... with a factory stock 3.55 gear.
(Edit: factory gear for a three speed stick 55 Chevy was 3.70)
 
take the car out now, put it in 2nd gear and cruise 60 mph for 30 minutes. IF that drives you nuts, then you don't want 3.55's or 3.73's.
 
maldridgeks said:
Hello,

Recently purchased a sharp driver quality 1970 plymouth duster. Car has 318 with Edelbrock Intake and Holley 4 Barrel Carb paired with 3 speed 904 trans and 8 3/4 rear end with non-posi 2.76 gears. Rear tires are 255/60/15 and front tires are 215/70/15.

I am planning to convert the rear diff to sure-grip and upgrade the gearing. I am considering 3.55 or 3.73 and plan to mostly drive car in town but will occasionally want to drive short distances (30-45 miles) and would like to be at highway speeds up to 60mph.

Curious to hear if anyone has built a similar car and has experience with a build like this?

Thank you in advance.
Click to expand...
I'd go with 3:55's
 
All depends what your trying to do, performance wise I feel you should run like 2 gears deeper than people with a similar built 360 generally would, so a 360 like yours generally gonna be 3.23/3.55 so to me 3.73/3.91. If highway matters more or best 1/8, 1/4 mile or best 0-60 mph or 0-100-0 mph all gonna change what to run.
 
yikes.jpg
 
AJ/FormS said:
"Best gearing" is 4.30s AND with a 2800 stall, and use the family car for hiway.
But yur gonna need at least 275/295 tires.
But I gotta tell you something.
4.30 x 1.45= 6.24 ratio in Second and
2.76 x 2.45= 6.76 ratio for First.
The engine doesn't care how it gets these ratios.
These numbers are so similar that Second gear with 4.30s will feel almost identical to First gear with 2.76s. Therefore, this is trading away a really good cruiser gear (the 2.76s) for a really really good starter gear (the 4.30s)
IMO, the better choice is a 2800 stall and leave the 2.76s alone. Then install the 275/295 or bigger tires, and you may not need the Suregrip..
If yur not willing to install a 2800 Convertor, then
I agree with all those who point to from 3.23 to 3.73, will get you started.
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IMO, you need to come up to my place (~13 hours each way), and spend the night. The next AM, we can go to the shed/shop and dig out, everything you need which is a mid 70s A904 cloned to be the same as from a 71 Demon and then hotrodded, and it already has the 2800 in it.
Additionally, I can supply a preassembled center-chunk from 2.76 to 3.23, with a reconditioned Cone-style SureGrip. and
I can have a pair of New 295s mounted up on a pair of aluminum 15x10s, on a narrowed rear end, that should slide right under your Duster. I can even supply a pair of matching wheels for the front/no tires at this time, but could have ready by the time you get here.
Then you can take it for a spin to the drive-in at supper time, about 18 minutes each way, and see how you like it. Then we can sit around until bed time and you can stay in the spare bedroom again. But if you don't like something, next AM, we'll just back it into my little shop and change it.
I'm retired now so you can come anytime, stay as long as it takes.

Or if you're into 4-speeds, I have a complete kit (less the floor hump and less the under-dash reverse light) that I might let go. This works well with a set of 3.55s, that I have, lol.

Yeah, I know, you only asked about gears, lol.
Click to expand...
AJ, Send him a pm. I have to do my moderator duties and delete your post. No selling in open forums. Sale and wanted forums only.
 
3.55 is my preference. Had a 318 with a 68 340 cam and 70 Sixpack hooked to a 727 with a 2800 stall. 14mpg around town, 18 on the highway. Oh yeah, that was in the little old 69 Fury ragtop that I had for 17 years.
 
Since it ain't cheap to set up new r&p's these days, i would base your choice on if you plan on any future upgrades. I ran a mild engine combo with 3.73 and 27'' tires as a daily driver with a average around town speed of 45/50mph and only several 150mi trips to the beach that were kinda aggravating. That combo puts your rpms just under 1/2 the mph, like 60 = 2800ish. It sounds like that is livable to you, but with that said and no plans for a bigger cam or a racier setup, i'd probably go in the 3.23 range. That would be plenty peppy for what you have now.
 
As stated above, depends on your goals. Unless you plan on going on with a larger cam than stock and emphasizing out of the hole acelleration, I'd keep your existing gear. You can always find another 8 3/4 chunk and build it with 3.73-3.91 gears. That way if you want the highway gears back, there available as a spare. I had 3.91 gears , 255/60/15 rear tires and mild stall behind a mildly cammed 318 build a few years ago. Super acelleration, but 2800-3000 rpm at 65mph gets old real fast!
 
maldridgeks said:
Hello,

Recently purchased a sharp driver quality 1970 plymouth duster. Car has 318 with Edelbrock Intake and Holley 4 Barrel Carb paired with 3 speed 904 trans and 8 3/4 rear end with non-posi 2.76 gears. Rear tires are 255/60/15 and front tires are 215/70/15.

I am planning to convert the rear diff to sure-grip and upgrade the gearing. I am considering 3.55 or 3.73 and plan to mostly drive car in town but will occasionally want to drive short distances (30-45 miles) and would like to be at highway speeds up to 60mph.

Curious to hear if anyone has built a similar car and has experience with a build like this?

Thank you in advance.
Click to expand...
I would go with a good all around 3.23. Your rear tires (is what matters on gearing) are already low, so I would stay away from the ones you are thinking about. Years ago I installed 60's and then 50's and had to change my rear from 4.10 down to 3.55.
 
I went on the maiden voyage in my 74 Duster project with a mild cammed 318/904. I put in an 8.75 with 3:55 Sure Grip amd the other day I got a chirp backing out the driveway and then after a couple trips around the block, I reved it up and laid a little patch just to let the neighbors know I was done
 
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maldridgeks said:
Hello,

Recently purchased a sharp driver quality 1970 plymouth duster. Car has 318 with Edelbrock Intake and Holley 4 Barrel Carb paired with 3 speed 904 trans and 8 3/4 rear end with non-posi 2.76 gears. Rear tires are 255/60/15 and front tires are 215/70/15.

I am planning to convert the rear diff to sure-grip and upgrade the gearing. I am considering 3.55 or 3.73 and plan to mostly drive car in town but will occasionally want to drive short distances (30-45 miles) and would like to be at highway speeds up to 60mph.

Curious to hear if anyone has built a similar car and has experience with a build like this?

Thank you in advance.
Click to expand...
I had 355's with a 904 behind a 318 in my 66 Dart Conv. I changed to 323's more driver friendly, I have 355's in my other Dart with a 4 speed that works out very well
Joe
 
I don't get this gear for the highway, every time someone ask about gears, be fine if the OP said I'm gonna do a lot of highway or highway is important to me, he said the odd time he might do a 30 minute jaunt down the highway, I'd go 3.73 gears a 318 will love them, 3.55 ain't gonna shave off that much highway rpms. Your gonna feel the increase of a full ratio from 2.76 to 3.73.
 
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