Rear gear choice

m0par0rn0car

m0par0rn0car

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I have a 1969 Plymouth fury 3 and it currently has a 318 and 727 with 2.91 gears. I've put a little work into it and the car runs and drives great right now but my girlfriend's uncle sold me a 1976 crate 440 and a brand new 727 for $500 and I couldn't turn that down. I don't really want to do the engine swap just yet because of how well it runs so my plan now is to run the 318 until it blows up. The car is my daily driver 90% of the year and I beat on her pretty good. It has a 8 3/4 rear end but with the stock open diff. I'm planning on swapping a posi into it at some point but I'm not sure what gear to go with. Currently I'm trying to choose between 3.23 and 3.55. On the one hand I'd like the performance of a 3.55 but I drive on the freeway pretty often and want to be able to drive 80 mph without sitting at the top of the rpms. At the same time I'm going to run whatever gear I choose with the 318 and the 440 so at the end of the day I'm going to need what's best for the 440 even if it isn't really the best choice for the 318. Anyone have any experience with running either of those gears on a daily? I'm basically trying to go with the best middle ground of driveability and low end performance.
 
Ring and pinions fall in the same catagory as cams and carbs. In a street application, if you have two sizes picked out, error to the smaller side. It’s our nature to always go “just a little bit bigger.”
 
If your in the highway all the time 2.76:1 or keep the gear ratio you currently have now, these cars were designed for highway running first
 
3:23 works well with 275/60 15's



you would not be able to tell the difference between 2:91's and 3:23's unless you had a tach. It would only be a 245 RPM difference.
 
MEV said:
3:23 works well with 275/60 15's



you would not be able to tell the difference between 2:91's and 3:23's unless you had a tach. It would only be a 245 RPM difference.
Click to expand...


That's a lot of revolutions over 10000 miles.
 
2.9 is the sweet spot in my opinion. (Get it, opinion):lol:
 
detmatt said:
2.9 is the sweet spot in my opinion. (Get it, opinion):lol:
Click to expand...

If you like to run 80 MPH on the highway a lot I would either stick with the 2.91 rear gear. FYI, the standard, stock rear gear for a '69 Fury with a 318 was a 2.76.
 
At the end of the day, YOU need to do what YOU want to do. The engine/trans was a fantastic deal, but doing that swap can be involved and somewhat expensive (considering all of the 'little things" that would need to be changed between the existing motor and a B/RB engine, plus finding them, which is your "time"). Sure, they all went in the same place on the assembly line, BUT it was also a complete package of parts that you'll need to find to NOW make it work well, as it should. Much can come from donor parts cars, as some might need to be sourced from quality vendors. Not quite the complete, easy bolt-in deal just to change the engine family in any vehicle, by observation.

The desire to "run 80mph" puts the axle ratio choice back to pretty much what you have now. What torque converter is with the RB 727? Adding the SureGrip can be a good deal, even with the current gears, considering your location. Let THAT be the first change.

Right now, gas prices are in the lower end of the modern spectrum, but that won't last forever. Which means that a RB-powered car, how ever much it might "romp and stomp", could become a very limited-use vehicle in the future, whereas a performance small block that runs good and gets decent fuel economy on the highway can be a fun asset to have. Otherwise, you can easily take the Fury III out of the "daily driver" orientation, which will mean you'll need another "daily driver" to replace it, while keeping it around. Parking space, insurance, etc.

The OTHER things is what specs that 440 has in it! Cam is a big one, as can be compression ratio. Cam will dictate the operational rpm range, compression ratio can dictate the need for more expensive fuels. It might make lots of horsepower, but feeding that power can impact ultimate costs of doing so, by observation. If the cam is "too much", it will decrease lower rpm torque and then dictate the need for a deeper rear axle ratio, which will THEN impact the desired ability to "run 80mph".

Several things to consider. Sometimes, even attainable dreams CAN get expensive, at many levels. Having a "hot rod big block" and "daily driver", many times, don't really mix well, but that can be a variable situationi.

Many things to consider,
CBODY67
 
You would have no trouble running 3.23 gears with either engine. 3.55's are more of a stop light to stop light gear if you are into that. You should expect to be getting good fuel mileage with the current gear set on the highway. Going to 3.23 gear will drop that about 2-3 mpg. 440 running 3.23 will struggle to much over 10mpg with a stock carb and less with a performance cam and carb. All depends on what you intend to do with the car..

Dave
 
2.94 behind a warmed over 383 bored .40 over 268 Comp cam shift kit and light stall..
Tons of highway at 70-80 MPH all day.
My point is the 440 will have more than enough torque to turn the highway gears.
As long as you do not run out of highway and how big your balls are you just might find top end!! LOL!!

750 miles 15 hours is just one of the "normal" drives the BoaB does,LOL!!
(From the FCBO show last summer Volo IL) Tim's NYer is now under construction..
cell phone june 2019 157.jpg
 
m0par0rn0car said:
do you ever drive on the highway? if you do how does it do on a longer drive?

I occasionally drive a couple hours or more and don't want to sit at 3500 rpm or something like that
Click to expand...
Oh yes. Drove to Dunsmuir, CA, to have lunch with Dave (Carmann) last Spring. Car runs well. With the 440 though, it will never get good MPG. Best I've ever seen is 15. Average is 12 on the highway.

I put 3.23s in it for a short while. Went back to the 2.76 gears, only dropped in a SG unit instead of the original one-legger. Other than a slightly quicker acceleration I saw no advantage to the 23s.

Picture is from a drive I took in March, '17. Drove over 300 mile round-trip one Friday. Had a ball!

20170317_132222.jpg
 
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I think I'll probably keep the stock gears for now once I get the posi and see how things go with that. I suppose it's not too much work if I change my mind in the future but for the driving I do it sounds like I should be good with the 2.91 for now
 
my '65 300 383-4 came with 3.23s. hated 'em. like driving in 2nd gear. swapped in 2.7s from a '69. love 'em. car still takes off and absolutely screws down the turnpike.
 
i agree with @detmatt i think 2.91 is the dream gear for a C Body if used for manufacturers intended purpose. Groceries, road trips, towing, etc.

Especially with a 318 I personally wouldn’t dream of going to a 3.23 or 355. My Fury has a 2.76 and it already can be a dog in the far top end. Especially if you’ve ever tried to keep up with some of these new cars on a major expressway like I95, I84, I91 where the slowest car is going 70+MPH you know that sometimes you might need to overtake someone quickly on the top end. But hey It really depends on what you’re doing with it, and what vision you have for the car.
 
For reference, a large portion of modern vehicles come with a final gearing of somewhere between 2.7-3.1 so don’t feel like you have to go too big!
 
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