What rear gear should I go with?

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BlueDream

BlueDream

Aspiring Mopar enthusiast
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I've got a pretty mild 318 in my '73 Dart but I've still got the stock 8 1/4 3.21 geared open diff that came behind the /6 when it left the factory. The rear end has never been rebuilt and has about 250,000 miles on it. I'd like to rebuild it before I blow it up and I want to go with a 3.55 or 3.91 gear but Im indecisive. Would a 3.55 sure grip be a big step up from the 3.21 open? Or should I just go ahead and get a 3.91 sure grip? The car stays in town for the most part, would only see the highway a couple of times per year. I've got 295/50r15's out back and an A-230 three speed. Any input on what to go with would be great
 
Since you know what the 3.21 feels like with those tires, I would go with 3.55 gears. If you go too low and may not like it, it's a waste of time and money to yank them out and try again. But it's what you're willing to deal with on the few times on the highway
3.91 gearing is close enough to 4.10 to just go to that route if you decide the lower gear.
 
The 3.21's just don't push me back into the seat like I want em too. Would I be able to feel a good difference with 3.55's?
 
I really am pleased with the 3:73 gears I put in mine. Though they were never a factory option, I really like the combo of driveabilty and performance. Mine had 4:56 gears and while it was an absolute blast for short runs, it really sucked to try and actually drive it anywhere more than a few miles away from home.
Not sure if that gear combo is available for your 8 1/4 or not. I don't think you will gain the 'feeling" you are after by going to just the 3:55.
I am quite sure you would enjoy the 3:91 setup since you say it won't see much highway time.
I would look up some charts with gears and tire size to see what the RPM ranges would be at 60mph.
 
MobileCustoms said:
I really am pleased with the 3:73 gears I put in mine. Though they were never a factory option, I really like the combo of driveabilty and performance. Mine had 4:56 gears and while it was an absolute blast for short runs, it really sucked to try and actually drive it anywhere more than a few miles away from home.
Not sure if that gear combo is available for your 8 1/4 or not. I don't think you will gain the 'feeling" you are after by going to just the 3:55.
I am quite sure you would enjoy the 3:91 setup since you say it won't see much highway time.
I would look up some charts with gears and tire size to see what the RPM ranges would be at 60mph.
Click to expand...

I'd be at about 3,000 RPM at 60 MPH with 3.91's. With the 3.21's my engine is turning about 2,500 RPM's at 60. I was able to find some 3.73's but they're $325 and thats more than I want to spend on just a ring and pinion. I'm hoping to score a sure grip with gears for around $300.
 
$300 wont buy much push.
-Maybe a pick-a-part 360. Bam. Instant 14% everywhere. You will need a matching flywheel, a pilot bushing, and a driverside mtg bracket. It will accept all your go fast goodies, including the entire top end.You can keep the 3.21s.That 360 will easily equal 3.73s or better. And 3.21s are such a pleasant 2nd gear, in town.
-With the 318,and 3.7s or 3.9s and only 3 gears you will be forever hunting for a gear. Better to back engineer.Select a rear gear that will put you in a trans gear and rpm, at your favorite cruising-around-town speed.Say you cruise at 33mph +/- 3mph. Say you want to be in second gear.Say you like the way the engine responds or sounds or is happy at 2500rpm( pick your own parameters). Doing the math for 2.95s at 27.5 tall;What would that work out to? You see how that works? Now you are in the "perfect" gear.
-If your engine doesnt have the grunt youre after, more CID may be in your future.Or shed some weight. Every 100 lbs is about 10 hp, at the top end anyways.Its quite noticeable on take-off. Steel 15x10 inchers are HEAVY.One weighs more than 2 aluminums.And they hammer over speed-bumps and through potholes. 295s look sweet, but they are heavy. Who sits in your backseat? Mine went years ago.Mufflers weigh a bunch. Now, you gotta have at least one, but it doesnt have to be a brick. Center console; gone(that thing is surprisingly heavy. Heater;35#-gone. Rear window regulators? Well, with no rear seat, who needs em. 200lbs is like 20 hp or about a cam size. 200 lbs is doable. After that it gets tougher, and/or pricier.
-If I were to recommend just one thing it would be a change to a 3.09low 4spd.Not the o/d one but the direct 4th from the early As. It will pull all your rpm-drops up, and give you a nice starter ratio that 318s need.Again, back-engineer for the new cruiser-gear.
-A bit of math here,now, could save a good bit of money, time, or shoulda-couldas.
-Oh and BTW, back in the beginning I said the 360 would accept your entire top end. I wasnt kidding. I once put an entire 318 2bbl top end on a 340.Stuffed it into a 65 Valiant wagon, with fenderwell headers. That thing was a blast around town. Didnt take long to blow up the 904 though. Ahhh, the memories.
-Wishing you every success.
 
Buy good high quality gear sets! For some reason the aftermarket for 8.25 & 9.25 rears the qualiy of them is junk! (Not lapped in properly) & noisey! Ask how i know! I had 2 sets of yukon 3.55 gears sets & 1 US gear set from randys & ended up installing a salvage yard set from a dakota that were perfectly quite afterwords. Pretty sad when even the sales rep ive delt with for years even automatically recomends factoy mopar gear set due to the many "noise" complaints from aftermarkets! Seems they got the 8.75 gear sets right on,but not the most common diffs now days i guess! The 4.10 richmounds i put in my old drag dart set up good & were quite. Weird but worked good for richmonds.
 
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