3:55 gears to 323 gears

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erafuse

erafuse

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Have a 1969 Dart Swinger 340-4speed 11.1 compression hydraulic cam .510 lift 245/60/14 tires 3:55 gears. 2900 rpms at 60 mph mostly highway driving. Would there be much difference in rpms and gas mileage going to 3:23 gears at 60 mph? Any info would be appreciated thanks.

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About 9%, or 2640 rpm at 60 mph. I'd imagine fuel savings would be dependent on just how efficient your engine is at a given rpm.
 
Everyone has their opinions, so here is mine just for fun....

With 14" tires, I'd want a 3.23

With my bigger cars 3.23 is also a good choice with B and C-bodies with 15" tires.

For a performance B or E body I like a 3.55 with 15" tires.

I have thousands of happy miles on my cars with those combinations.
 
The difference will be quite noticeable.

I just swapped in a 3.23 for a 2.94 in a B-body with 15" tires. I noticed the difference just backing out of the garage! :)
 
Thanks for the info. So turning 2600 rpm at 60 mph verses 2900 rpm would probably be better? Speed limit here is 50_60 mph not sure if it’s worth the bother.
 
If your exhaust is loud and drones at highway speeds, 300rpm can make a difference to you. Probably in the grand scheme of things, with stock exhaust, 300rpm isn't that big a deal imo and not worth the expense.

If it were me and I did mostly freeway driving, I would swap them out.

I have 3" exhaust with Flowmaster 40's and 4.10 gearing. It kills me on the freeway, but around town is a hoot! I'd kill for a 3.23 chunk. Well, almost.
 
Tinmannz said:
If your exhaust is loud and drones at highway speeds, 300rpm can make a difference to you. Probably in the grand scheme of things, with stock exhaust, 300rpm isn't that big a deal imo and not worth the expense.

If it were me and I did mostly freeway driving, I would swap them out.

I have 3" exhaust with Flowmaster 40's and 4.10 gearing. It kills me on the freeway, but around town is a hoot! I'd kill for a 3.23 chunk. Well, almost.
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2:5 TTI exhaust Dynomax super turbo mufflers at 60 turning 2900 rpms it seems like it’s turning more and it’s loud lol.
 
Our family factory-stock special order 1971 Plymouth Road Runner with G60-15 Goodyear Polyglas tires, torqueflite, and 3.23 open 8 3/4 differential would run exactly 3,000 RPM at 70 MPH, so around 2,500 at 60 MPH. A four-speed should show a little less slippage than a torque convertor equipped vehicle (lower RPM). Photo is my parents old RR that I found recently via Facebook & FBBO:

GSA road runner 2011 Mopar in Park Minnesota.jpg
 
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I installed a 2.76 in my ‘69 Dart 340 A727 with a 245/60/14 tire. Love those gears for highway cruising.
With a 4 speed you have to ride the clutch a bit to get it moving. No problem at all with the automatic transmission installed.
 
erafuse said:
2:5 TTI exhaust Dynomax super turbo mufflers at 60 turning 2900 rpms it seems like it’s turning more and it’s loud lol.
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At one time, in my 360/4gear/3.55s/68 Barracuda, I ran the Mopar 292/.508cam (.543 with 1.6 arms) @ 11.3 Scr.The car had a TTI dual Full-length 3" system also with Dynomax muffs. But I bought mine with turndowns that I ran to just under the bumper : and yeah it was a lil loud. No drone tho.
The math says 2870rpm@65mph, so you are spot-on. . I drove 56mph@2470 (3.55s)on short hops, but wore earplugs on trips over 20 minutes. I tried 3.23s but the cam did not like those.
I sold that big Purple as fast as I could. It just wasn't For me.
The next cam, a 270/.538, in at 10.70Scr now, was no quieter.
The final cam, a 276/.549, in at 10.95 now, is also no quieter.
In the end I bought a GVod which brought the Cruise-Rs down to 2240. It is amazing how much quieter the exhaust is. I threw the earplugs away.

As to fuel economy, it's been written that for every 1% decrease in cruise rpm, you will increase your fuel economy by half a %. So from 3.55s to 3.23s should be good for ~5%. But there is a limit. If your rpm gets down into reversion, then your economy will turd,fast.

You can find your minimum cruising rpm by just reving the engine up in Neutral and observing the manifold vacuum. The lowest rpm with the highest vacuum is it. When you find it, pull in some timing until the Rpm peaks. Then back up the rpm a few hundred, and slowly run it up again until the vacuum peaks, again; There it is. That is your minimum best cruise rpm and timing for it.. On the road, you may find that the engine is ok or even likes just a lil less timing than it did on the in-Neutral test.
Most guys are 15 to 20, or more, degrees shy, of optimum cruise timing. The sound and loudness of the exhaust, will mellow out, as the fire no longer burns in the pipes.

BTW
if that 11/1 engine of yours is aluminum headed, like mine;
I found a 223* cam waaaaay more fun, and it got 4cylinder type fuel-economy with an overdrive. The next bigger cam (a 230*) had a bit more topend power, but traded away a bunch of low-rpm torque to get it; and the fuel-economy was gone. IMO it was a bad trade. I solved the softness with a 3.09 gearset from the Commando box. I worked on the fuel-economy for 10 years but never could touch what the 223* cam had got. Both with same gears.
The reason I mention this is because that big cam of yours will not like 3.23s much, never mind less. Whereas my 223* cam liked anything, even down to 2.76s and as a100% streeter the 223cam was just as potent in the first two gears;
and 3.55s go to 82mph @7000,in second gear,
so IDK what more I could ask for,lol.
I have never missed that 292/509.
 
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If you have to buy new gears or carrier, you could just go with taller tie. A 28" diam tire like 245/70R15 has same width and would reduce 200 rpm at cruise.
 
You did say you have 4spd, right? IMHO, stay with the 3.55s. Ma Mopar put 3.55s behind 4 spd SMB. and 3.23s behind automatic for the difference in RPM at highway speeds. Also in your case as mentioned your camshaft. I had a noisey set of 3.55s behind my 340 auto, 10.25 cr 474 cam. I had a nice 3.23 set I swapped in to take care of the noise problem. Actually quite a difference in excelleration performance. And car used to bark second very easily at low rpm, had more neck snap. The trade went to the highway performance end. Higher cruise speed, with more pedal for high speed bursts, cooler, consistent engine temps slightly better gas milage. But if I had a fourth gear like you, I would think it would be the 3.55s for sure.
 
What are you spending a year on gas $500 $1000 even $2000? say you save $200 a year at best does it really matter?
 
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When you say highway driving, do you mean 60-65 or does this mean interstate driving 75-80? If just 60-65 3.23's are good. If you do extended interstate at 75-80 2.76 are better, that is without an overdrive. My 71 Polara in my avatar had 3.23's, went to 2.76 behind the 440. I knew I would be on the Interstate a reasonable amount. I could run 90 all day if it wasn't for the patrolman.
 
Fill up and go like 20 miles on a highway at 60 mph with your 3.55 stop top up and come back 20 miles at 55 mph and top up which will rev like 3.23 at 60 and calculate the difference.
 
In cars with high gear ratios when ordered with 4 spd the flywheel weight is much heavier. Most 323 355 391 cars came with the same weight flywheel. Buying and stripping these cars since the 80's I have found that 4 spd or 3 spd cars ordered with 294 or 276 gears came a heavier flywheel.

I had a 340 swinger that I had bought in the 80's . I was the second owner so I new it was never touched. It had 294 gears and The car was like an airplane .loved driving that car on the highway These heavy flywheels are flat on both sides and look as though you can put them on Backwards they are much heavier. I have one here and many of the lighter style . I am keeping the heavy flywheels for cars with big cams and higher ratio gears. I can check the weight if interested in knowing the difference.
 
273 said:
Fill up and go like 20 miles on a highway at 60 mph with your 3.55 stop top up and come back 20 miles at 55 mph and top up which will rev like 3.23 at 60 and calculate the difference.
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Hang on 273.
That puts the 3.55s pushing more wind , which is gonna take more throttle to do; so IMO not a fair fuel-mileage test.
There is a long standing ratio that says a 10% reduction in rpm, can result in a 5% reduction in fuel mileage. And vice-versa.
 
On another note
But honestly, the biggest hindrance to fuel-economy is not having enough ignition timing, and or having the exhaust valve open too early, made worse when both are in effect; and/or operating in the wrong rpm for the specific cam.
All the math in the world goes right out the window, when trying to make a 292 cam operate at 1600rpm, it will just spew unburnred gas into the headers, and if/when it catches fire in there, it will make it all the worse.So some common-sense has to prevail.
Generally, I have found 2200 to be the lower limit of a hot street cam, and the engine will want more ignition timing there, than is generally possible to give. Mine has liked up to 60 degrees there.How can a guy meet that need? No stock or modified parts can achieve that. At the best; if you cranked the mechanical to 38* and modded your Vcan to get 22*, there is your 60*.. But just try and drive that hummer at 2200 at WOT, with 38* of lead. Furthermore; what will the idle timing be with such a set-up?, and what will be the idle-speed? Yur not gonna successfully marry this tune to your street engine. You will require a stand alone timing computer.
If you don't give the engine the lead it wants a cruising speed, fuel-mileage will suffer.

It's all about maximizing the cylinder pressure at just the right time of crank-rotation, to maximize the energy-transfer of the expanding exhaust gasses, to the crank and to the rear wheels following.
So the key player to good fuel-economy, is the running cylinder pressure, at cruising rpm..

This is how, the stock lowly LA318 can make fuel economy with it's atrocious sub 8/1 Scr and subsequent meager cylinder pressure of barely into the 130s. With 2.76 rear gears the rpm mathes out to 65=2230 in loc-up. The peak efficiency of that LA comes at about 2400rpm.
That 318 cam has specs of
240/248/112+4/Ica of 48*/132comp/120power/ Effective overlap of 16*
Check it out;132* of compression from an Ica of just 48*. That traps a lotta aircharge, preventing it from backing up into the intake. The meager overlap means very little,if any, exhaust dilution. But the thing I really want guys to see is the 120* of power extraction. That is a lot! And by the time the exhaust exits the chamber, there is not much energy left in it.
If you get the ignition timing right, you can impart a lot of pressure to the short 3.315 stroke, which will be down to 2.89effective. The cylinder pressure is expected to be 140 at sealevel, with a VP of 116.
VP of 116 is what most of us are used to seeing/feeling with our 318s, and more often than not , with 2.76 rear gears. We know how that feels; albeit with a 2bbl and single exhaust.
If you assume a 7/1 pressure rise, due to heat, then one could expect a new engine with 140 x 7=980 pounds of pressure at WOT from this engine during it's peak efficiency. That's not bad.

But if you bump the Scr to 9/1, then the cranking cylinder pressure might rise to 160psi, and at the same 7/1 expansion ratio, this now mathes to a potential of 1120psi at WOT; a 14.3% increase. As to torque, this is like an across the board increase of 14.3% in rear gear or from 2.76s to 3.15s. Or in terms of peak power, about 20hp.
Now that is something; same engine, only the Scr was increased.
But what is the first thing a newbe says? He says; "not taking the short apart". And most of the time that 40 year old engine is still on the original rings! With gaps you can drive a truck thru, and the rings are coming into the hole under the ridge and spewing compression straight into the crankcase. You know; 980psi leaking thru that monster gap.

So he puts a too-big cam in there, and the cylinder pressure falls to 120 , and 120x7=840 running at WOT, so the torque has fallen to 86% so it feels like the 2.76s have turned to 2.37s ....... and here he comes with "what do I do now?", and the only solution is a higher stall and bigger gears.
In the meantime, the fuel economy is so low, he dares not drive it anywhere.
To my thinking this is a ridiculous thing to do.
If I wanted performance from a 318, the very very first thing I would do is bump the pressure up. And that is what the factory finally did when the 5.2M came out.
 
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AJ/FormS said:
Hang on 273.
That puts the 3.55s pushing more wind , which is gonna take more throttle to do; so IMO not a fair fuel-mileage test.
There is a long standing ratio that says a 10% reduction in rpm, can result in a 5% reduction in fuel mileage. And vice-versa.
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Yes it's not perfect but gives an idea, doubt the difference be all that much. If your 5% is right say he get 15 mpg now that would be 15.75 mpg after even 10% would 16.5 mpg. You know how much highway driving for that to equal any real $$$ in the pocket.
 
AJ/FormS said:
If you don't give the engine the lead it wants a cruising speed, fuel-mileage will suffer.
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THIS ^^^^ is by FAR the most important sentence in your entire lecture.

So many guys say they don't or won't run Vacuum Advance on their engines on the street, because so & so said not to. Or they read somewhere that you get better performance and/or HP without Vacuum advance hooked up. That is for RACING ONLY, which is WIDE OPEN THROTTLE only.

I curve my distributors for about 15-18 initial timing at 800 rpm, with mechanical going to about 34-36 at 2000-2200 rpm and finally at 54-56 degrees total advance with the Vacuum advance hooked up for cruising speeds. My cars all perform very well like this for street cars.

Of course a computer controlled ignition setup would do a better job of managing advance at all engine speeds, but that defeats the purpose and enjoyment of our classic mopars.

Anyone is welcome to agree or disagree with my setup as you wish, it isn't going to change my mind on how I run mine!
 
Without my knowledge, the vacuum advance during my road trip in the '65 Formula S (318 auto) was disabled and MPG were very low. Ran out of gas in a very remote area of Mexico, just having passed some people selling fuel in 5 gallon containers on the side of the highway. My spouse was following behind me, so she turned around and bought some gasoline at black-market prices.
 
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