RPM at 70MPH

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1973Barracuda

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Those with hemis, what rpm are you running on the freeway?

I am trying to nail down my TC and rear gear. Ran an rpm calculator and with a 3.55, with 17 inch tires I will pull 2300rpm. I figure a few hundred rpms of slippage from the TC I am a bit over 2500.

I think it will be a good place to be on the freeway, thoughts? This is a driver that will see limited track duty.

3.55 rear
46rh
25 inch tire height

Thanks
 
Put some taller rear tires on to knock down your rpm a bit if you want. Anything under 3000 is doable, just depends on how far you want to drive it. I am putting a 28" tire on a Duster with 3.55's and a 2000-2200 converter, hoping it stays under 2500 at 65 - no hemi. Should be a good cruiser, sounds like you are about right, keeping the rpms in that 1700-2200 cruising range @ 60-65 is kind of the ideal.
 
Since I have not driven a hemi, I don't exactly know what it is "comfortable " at rpm wise. I don't want to get it too low and create a dog at freeway speeds. Not sure if it has a "happy" range for cruising. If that makes sense.

Thanks for the input BTW

I am in Afghanistan trying to buy all my parts to finish up in February when I get back.
 
Be safe brother, Merry Christmas! Any hemi won't be a dog on the highway anywhere around 2k rpm. Don't wind it to death at 3k rpm cruising either. Thank you for your service! Sure some hemi guys will chime in soon enough
 
Merry Christmas to you as well. Good info on the rpm. I have an open 8 3/4, need a SG. Oddly enough, there is a 3.55 for sale on this site......
 
BTW, I am not active anymore. Retired December 2014, contracting now. Can finally grow a beard. But, oddly enough, still not enough time or money for my projects.....
 
To help with selecting a happy cruise rpm, I do this;
With the engine fully warmed up, and the timing set right/bugged out,and the tranny in Neutral, and a vacuum gauge hooked to manifold vacuum; I slowly bring the rpms up, while watching the gauge. The lowest rpm that makes the highest vacuum I consider the lowest happy cruise rpm. Happy, in the sense that the engine is finally out of reversion, and becoming efficient.
Now that I know this,I take it out for a cruise. I get it into high gear. I start at 45ish mph, on a flat level hi-way, on a windless day.After the mph becomes stable, I mark down the speed and vacuum reading. I continue in 5mph increments. Sooner or later the engine vacuum will peak at a high number, and sooner or later the vacuum will begin to drop, as she works harder to overcome wind resistance and chassis friction. Somewhere between that maximum peak vacuum and the point at which the vacuum begins to drop, is the happy place.If I was to then marry the two,with gears, I will have a vehicle that has the best potential for fuel mileage.

-Remember the first test was to find the lowest happy rpm, while the second was to find the highest rpm.
With a stock Magnum, this rpm could be pretty low. I had a 92 TB-injected-teener Dakota that was very "happy" at 2200@70. This was with 3.91s, the A-518, and 28" tires.In a lighter chassis, I'm sure it woulda been happy at a much lower rpm.
With a hemi, the same principals still apply. But since it is an inherently better design, I wonder if the minimum cruise rpm will come in even lower.

Performance is another story.
But with the O/D tranny, You can run almost any rear-gear.

Personally I run a 367cid with 3.55s and a GV overdrive. I runs 2400@70. I find it a little busy, especially on long trips. 3.23s would bring it down to 2200 which by the above tests would be the minimum recommended for my combo, cuz that's where my combo vacuum peaks. And my combo would pull 3.23s ok as well. And I have a chunk ready to go. I just rarely cruise at more than 60, and the 367 is very peppy with 3.55s, and mostly, I'm getting lazy in my old age.heehee
 
Just clarifying! My pleasure.

Things are drastically changing for those following behind me. I am very glad I did my time before now. And it was getting bad on my way out. Getting worse now.

Keep thanking those still doing it, not much thanks or praise from the current regime. Off my soapbox now......
 
I'm putting a 5.7 with a 545RFE in my Duster, and have 3.91 gears to run in the 8 3/4. Even though the new Challengers come with 3.90 gears, and get good gas mileage....I keep second guessing myself about the 3.91's. I keep thinking I'd rather have 3.55's! :banghead: Then I just remind myself that the Duster will be a thousand lbs. lighter than a new Challenger so I'll quit thinking about it for a while!
 
You can do the math, but the 545RFE has a 0.67:1 5th gear and will live nicely with a 3:90 gear.
 
No Hemi, but my 318 Dart runs 2400 rpm at around 73mph with 3.55 gears and a 46RH (non-lockup). 17" wheels.
2400rpm happens to be exactly around the peak torque number of the engine.
 
I'm putting a 5.7 with a 545RFE in my Duster, and have 3.91 gears to run in the 8 3/4. Even though the new Challengers come with 3.90 gears, and get good gas mileage....I keep second guessing myself about the 3.91's. I keep thinking I'd rather have 3.55's! :banghead: Then I just remind myself that the Duster will be a thousand lbs. lighter than a new Challenger so I'll quit thinking about it for a while!
Per this, the 3.91 rear is with the 6 speed manual trans with a 0.5 6th OD gear. Our 2009 6 speed does 1750 RPM at 72 MPH.. and pulls down 25 mpg. The low RPM is the reason for the mileage.
 
Thanks or all the input. This was a huge impact on my decision. I was initially going to go the wrong direction. I thought I was going to go with a 3.73, or 3.9ish.

Figuring in TC slippage into my rpm equation, I will probably go with 3.55 or 3.23

3.23=2300 at 70

Again, thanks!
 
Thanks or all the input. This was a huge impact on my decision. I was initially going to go the wrong direction. I thought I was going to go with a 3.73, or 3.9ish.

Figuring in TC slippage into my rpm equation, I will probably go with 3.55 or 3.23

3.23=2300 at 70

Again, thanks!

The 545RFE has a lockup converter. ie no slippage at cruise. How tall are your tires?
 
this is what my hemi is doing, but it has an 8 speed
 

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Hate to sound contrary, but what we have found with the heavier trucks is that the hemi likes 2100-2300 at cruise and gives the best fuel mileage in that range if running free, meaning not pulling extra weight. The lower rpms cause the engine to lug a little and needs more pedal to accelerate or just carrier itself at cruise. Of course this has a lot to do with the vehicle weight and aerodynamics which the trucks have working against them. With the weight being lighter and aerodynamics better the sweet spot will move down some. I would guess 2000-2200 would be the sweet spot at cruise. Anyway this is what I have found with my truck as I've modified it over the years. Unfortunately it is a poor example at the moment with several mods being done and tuning incomplete. Plus I'm now using a 46RH trans with a .69 over drive on 29" tires and a 4;10 gear. Assuming you are running short tires I would think the 3;73 or 3;91's would work great. good luck..
 
Since this is turning into a question and answer about cruise RPM @ 70MPH, I'll throw out my combination that I hope to have running this coming spring:

1971 Duster, figure 3500LBS with me, a full tank of gas and spare tire
3.73 8 3/4" with 285/35-18's
6.1 with ported heads that flow 348cfm @ .600", TTI headers
.219"/.223" @ .050 .571"/.574" @ 112comp cam
.70 5-speed overdrive

I'm hoping this gives me all day freeway cruising @ 70-75MPH. What do you think?
 
That looks like a 26" tall tire. At 60mph with a 4:10 = 2580. With a 3:73 it would be 2341 and 3:55 is 2228.

Looks to me a 3:55 would work for you.
 
I will be running 17s, so 25 on the tire height.
I have a two pin, so no lock up.
Also to those running a 46rh, are you skipping the radiator and going straight to your cooler?
 
I have it hooked directly to the cooler.
On regular short commutes it feels just luke warm to the touch.
I know that's not enough to really expell any moisture from the trans oil, but I try to keep an eye on it.

During colder outside temps I usually put a simple shield in front of the cooler to block the direct wind (floortile).
 
Don't remember what diameter tire, but I swap between something like a 235/60/15 and 255/60/15 (pure street vs. street slick) and with the smaller ones I used to turn 2500-3000 easy with my 3.55 rear gears and the old 4 speed. I've since swapped to a T56 with a 0.5:1 overdrive and now cruise around 1750 at 70 mph. It'll lug a bit if you're one a steep hill or lay into it hard, but that's what the other gears are for. I don't think you'll hurt one cruising even at 3k, but it's a little nicer on the ears and peace of mind to bring it down. New engines are pretty amazing with longevity and taking punishment. Have taken apart 1 with over 250,000 miles that didn't even need to be bored and another with 160k+ that regularly ran with only 1 quart of oil in the pan that only needed to go .020 over.
 
That's the weird thing about these hemi's. You can run them at rpm's that would make you cringe if it were a 440, but with the hemi it seems like its no big deal.

Just for another reference, at 70 mph my 5.7L backed by a T56, 3.91's, and 31" tires runs around 3000 rpm in 4th, and drops to 2200 in 5th, and 6th is mostly unuseable below 80mph because the rpms are just too low.

I like my 3.91's, but with my first gear ratio they are on the ragged edge of streetable for my taste (can be jerky if you dont ease on and off the throttle). Your transmission has a first gear that is not quite a steep (2.45 yours vs 2.66 mine) so that would be helpful in your case. If you think 3.91's might be too much gear and you need to buy a new gear set you might want to consider 3.73's if you can find them.
 
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