Rear End: 3.23 one-tire-frier to.....3.55 Sure-grip?

[kawahonda]

Yep, I have a 3.23 one-tire frier.

School me on what I need to prepare for.

I'm thinking 3.55 gears should be perfect for a 340, even when it gets up to 350-375 at the flywheel. Allows it to drive on the highway/interstate and allows for some low-end fun.

Is this as simple as dropping the original center section out, putting new one end, and boom, done?

Most cost effective way is to probably scour local adds on craigslist?

There's a guy selling complete 8 3/4 center sets for $350 with gear of choice. "Sure-grip" is extra, but noted it will be a "deal".

Curious on your guys's thoughts/opinions. 3.55 a nice all-around gear? "Sure-grip" the way to go for almost entirely street?

[Dakota]

I can't speak to the gear ratio just yet.   I have a 3.91 open rear along with a 4 speed overdrive tranny for my 340, but the car's not on the road yet.   I'd like to have a Sure Grip too but it's on the "later" list.  Most of what follows in answer to your questions falls into that annoying category of "it depends".   

There are a couple of calculators available to figure out what RPM you'll be turning based on gear ratio and tire size for a certain speed.   Here's one:  https://www.randysworldwide.com/calculators/rpm/

Using this calculator for a 25.7 inch diameter tire and an automatic tranny while switching the differential gear from 3.23 to 3.55, RPMs at 60 mph would go from about 2600 to 2850.   If you're not on the highway much, this increase might not matter much to you.

If you have a properly set up spare 3rd member, it is pretty much a drop in.   The rear axles have to be pulled out a bit, drop the drive shaft, drain the oil out of the differential, replace the 3rd member, reassemble and enjoy.   

I've read a number of posts over the years about people getting a great "deal" on a used 3rd member, only to have problems with noise or some other issue because it wasn't set up right.  As with so many things on these cars and elsewhere in life, dealing with someone reliable/reputable is usually the route to the best outcome.   

If the plan is to change the gearing on your existing 3rd member, that gets more complicated but it can be done.  There are a number of good YouTube videos on the topic that can be referenced if that's a route that interests you.

Cass at Doctor Diff (www.doctordiff.com) gets a lot of positive praise as a source for parts and knowledge on these topics.

[73chalngr]

Is your car an automatic or manual. ? That will make a difference . I have a 727 auto with 391 sure grip and 60 mph is about 3000 rpm(wish I had that fourth gear sometimes), also I think most 340 autos came with 323 and 4speeds 391. I would get sure grip but there are two different styles out there can't recall the numbers but one grips better for racing.   Swapping them is easy just leave some nuts on center section and wiggle it loose some to drain oil.

[anlauto]

I would avoid buying used stuff unless you know exactly where it's coming from.

[Brads70]

3:55 is a good all around ratio for a non OD transmission IMO I had it for 5-6 years in my car.
This site has a pretty good gear ratio calculator . You can even play around/change the transmission gear ratio's to give you an idea what an OD transmission would do.
https://www.e-bodies.org/resources/rpm-speed-gear-tire-size-calculator/

[Chryco Psycho]

Pull the axles & driveshaft , 10 nuts & the center section will drop out , slide the new one in set end play to .010 & you are good to go .
There are 3 carriers , 741 smallest , 742 strongest & 489 which is fine if the crush sleeve is replaced with a spacer & shims
Most Used sure grips are worn out , especially cone type , clutch is much better as they can be rebuilt , used gears sets are hit & miss , if they are reset correctly they can be fine if the pattern is bnot matched up correctly they will always be noisy .
Dr Diff is one of the best suppliers around

[303 Mopar]

I had 3.55's in my Challenger and have them now in my Cuda and absolutely love them!  Both are sure grip too. 

[RUNCHARGER]

A lot of people like 3.55, I don't. I find they rev the engine too much on the highway and don't pull as nicely as 3.91s do around town. I either run 3.23 or 3.91 typically.
Used gears/limited slip can be just a down payment on buying new stuff. You might get lucky but it's a less then 50% chance.

[70/6chall]

From a different standpoint here, and not talking a real high horsepower drive train. My 1970 Challenger with Slant6 and Auto (904 with B&M shift improver kit). It came stock with of course the 7 1/4 2.76 gear set. I changed it out to a 8 3/4, 742, single track, 3.23 gear set. Applying a power standpoint even with a warmed up Slant6, she pulls a lot better in everyday traffic conditions, on the freeways not high winding with low RPM's as before, and with the weight of the car 3200-3500 it still cruises real nice and gas mileage wise is still there. 3.55's would , in my situation, be the lowest I'd drop. I know it's hard to apply the same idea comparing a 6 to a V8 but the mechanical idea is still the same. My two cents here.    Thanks,   Al

[Rich G.]

Ive had 391s and I have now 323s in my 4 speed challenger and 5 speed Cuda. Highway cruising is good good on one and great on the 5 speed car. Never had 355s but I would think they would be the happy medium. If you're not racing or doing burnouts you won't even know it's not a suregrip. I thought someone on this forum had a 355 for sale.

[RzeroB]

Love the "one tire fryer ..." part of the title of the thread!! Never heard that term before ... think it's hilarious!! 

[BIGSHCLUNK]

For an AUTO(727) I'd sure think 3.23's are the way to go. Otherwise you can really be wound up at 70-75 MPH..............

[nsmall]

I have a mild built 340 and LOVE my 3:55 sure grip clutch rear end.   It has a 727 and is so much more fun vs the 276's I had in it before.  80% of my driving is in town and I think the 3:91's would be too much for a long road trip.  However, I always want to go faster so I wouldn't be sad if I had 3:91.

I paid $850 for a 741 case and dr diff new clutch SG and very good used 3:55's.  IMO expect to pay 700-1000 for a nice set up