How's it been for anyone driving a street rod or pickup with the 5.7 Hemi and a six-speed automatic trans? I plan to have a 3:55 rear axle ratio behind a 65RFE trans in my 56 pickup build and goal is to get better than 20 mpg on the highway at 75mph/2000 rpm. Is that realistic for anyone out there running a 5.7 Hemi? Plus, just say "yep, got a Hemi" still counts to me being a long term Mopar guy.

I'll be interested in the response to this.
I just finished up my 5.7 swap, but I have a 46RH, 3.73 gears.
Haven't been driving it that long, but looks like 19-20mpg at 75mph at 2500RPM

My son is a 5.7 Hemi guy. He has a 5.7 in a 57 Dodge wagon he has been driving a few years, but its mostly around town. He says he gets about 13 in town. The wagon has a 3:73 gear 8.8 limited slip rear.
he also has a 2010 Hemi Charger, ex squad car he drove back and forth between Freeport and Rockford, about 60 miles one way on a 4 lane highway. I'm thinking the squad probably has around a 3:55 gear, but I'm sure someone will correct me if I'm wrong. He always got 24-28 mpg @ around 75 mph. That squad is a lot more aerodynamic then your 56 is going to be though. He also has a 5.7 Hemi Durango chassis under a 90 Ramcharger, but he just started driving that consistently a month or so ago, and it mostly sees city driving also. He drives his stuff fairly hard.

19 mpg 70 to 80 mph on highway for my 55 Dodge with the 5.7. 3:55 year. No idea what city is.

I think the garden variety LX cars with a 5.7 have a 2.82. The SRT8 got a 3.06. This would be for NAG1 equipped cars.

Kevin

Seems for some reasons engine that can cruise in the 1800 to 2000 rpm range hit a sweet spot for internal engine friction versus driveline drag and aerodynamic drag, engine torque curve all seem to be optimal for decent fuel mileage. Much below 1800 the engine starts to lug under load. Much above 2250 rpm the internal engine friction load increases to use more fuel. An old girlfriend of mine drove a late 80's Olds Delta 88 four dooor with the GM 3.8V6, and on the highway at steady 2,000rpm that thing would cruise out 26 mpg.

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I’m turning 2200rpm at 75 mph

What trans?

46RE out of the 2003 Ram I got the 5.7 from.

I'm planning to use the 65RFE trans in my 56 pickup build. The 65RFE features a lower 1st gear ratio of 3:1 and the final overdrive of .67:1
That'll give a bit more snap off the bottom, two more gears between to stay in the optimum rpm range and a slightly lower final ratio for highway cruising. A 3:21 rear axle would further drop the revs but then I'd have to give up some towing capability. If I could choose 5th gear for towing then a lower rear axle ratio would make sense. But the 65RFE trans doesn't allow specific gears for highway driving.

No 3.55, my 2010 pursuit hemi has 2.65 and I believe a 3.06 in the V6. I average around 18-20mpg when not getting into the peddle but I have the NAG1.

the only thing i concern myself with, is the rpm at cruise speed isn't too low, causing the transmission to shift in and out of overdrive, plus causing the engine to lug.
i have proven to myself a slightly higher cruise speed, along with the correct ignition advance curve, can produce more than just a couple of mpg's.
just an old man's opinion and experience over almost 60 years playing with this stuff.
your mileage will vary.

I think what we keep missing with these modern motors is the amount of HP and torque they are putting out, compared to what the motors of the 60s & 70s put out.

The lowly modern V6 puts out nearly 300HP in stock non-assisted power with torque ranges we never even thought about back then. Those HP and torque numbers are about equal to the old 383 2bbl motors. I don't ever remember being too concerned with any 383 not having enough power to drag a 4,000 lbs car from 1500 RPM up to 4,000 rpm, how about you?

The 5.7 has more HP then any Mopar motor other then a 426 Hemi in its stock non power assisted form. A 440 sure wasn't weak pulling from 1500 RPM to 4000 RPM, and that is what we are talking about.

So you hit the long drawn out monster hill and the 5.7 is humming along at 75 mph at around 2,000 RPM on the cruise control, but suddenly has to drop out of OD for a few seconds while pulling that hill, and you think its the end of the world? Where have all the men gone? Back in the day we would have mashed the gas and listen to the Holley howl with a smile on our faces, now we have become so concerned that our 345 HP motor can't take it because the 6 speed auto trans has to downshift, we have to whine and cry. How often do you really expect to encounter this event while your driving? If it does it more then you can stand, maybe you should turn off the cruise. With the cruise off, I tend to speed up a bit more when I know that big hill climb is coming.

Or, you could pull the auto and put in a manual trans. It won't downshift, might lug a bit before the computer adjusts the timing for a second or two, but it will eliminate the dreaded downshift. Or put the lower gear set in the rear and we will have to listen to you complain that it doesn't get 20+ mpg at 75 mph anymore.

Most of us, back in the day never had a cruise control on our 340 HP cars, and we sure weren't concerned about downshifting to go faster. We weren't getting 20+ mpg at 75 mph on cruise control, either.

With all that torque history in Mopar engines, it seems Mopar missed the boat with the 4.7 SOHC engine (though it is an engine Mopar got from AMC). Push rod or overhead cam in itself is not what determines torque. Torque is largely how an engine breathes and how that relates to a given rpm range. The Hemi head though, does breathe better, fills the combustion chamber better, which means more torque. Ford and Mopar is making big torque these days with V6 engines and forced induction. Ford with the Ecoboost engines make great usable low end power by forcing enough air/fuel mix into the engine. My company vehicle is a 2021 Escape with a 1.5L three cylinder Ecoboost making 197hp (though at motorcycle engine speeds of 6,000rpm), but it does make 214 ft-lb of torque at only 1,600 rpm. But it sounds rough as is typical for a three cylinder. But anyone who has ridden a Triumph Speed Triple, or a Laverda Jota, knows a three cylinder can make great sounds.

Too bad Mopar never developed a low inertia turbo system for the 4.7, something to boost the torque well into the realm of a Hemi. If I stay with the 4.7HO engine for my 56 pickup build I'll do the usual intake and exhaust mods. The 4.7HO makes about 310hp stock, so with a few mods may make about 325hp. Interesting reading here:

https://www.allpar.com/threads/4-7-liter-next-generation-corsair%C2%A0v8-engines-dodge-jeep.229964/#post-1085224062

For all the positives to read there (granted it is Allpar generated), it reads like the 4.7HO is the engine to have. Yet, many know the Hemi to be the engine of choice.

The 4.7 is a turd. The only reason that it doesn't get worse press is the 2.7 was made during the same years, and it is an entire septic tank full of turds..Can hardly give cars away that have either engine. They are the two nearest to nothing garbage engines ever built by Chrysler.

If you're going to sugar coat things we won't know how you really feel

It is not rpm, but “average piston speed”
and manifold absolute pressure that determines engine fuel economy.

It used to be with old ring materials, and cast iron bores, the average piston speed needed to stay below 1100 feet per minute.

Now with super thin rings made from even better alloys,
and special bore coatings,
average piston speed can be 1600 to 1700 feet per minute
and still get best fuel economy.

Throttle bores need to be about 60% open
to raise manifold absolute pressure to about 70% to 90% of 30 inches of mercury pressure.
A manifold vacuum gauge would show 4 to 8 inches vacuum at these levels.

100% absolute pressure and 0 vacuum would give good good fuel economy except that most of pre-2010 year engines are in “fuel enrichment” at that throttle opening.

Democratic long time MI Congressman John Dingell used to allow emissions law to have an “emergency passing power exception” that allowed auto engines to run rich and emit more pollution, and get less fuel economy past 80% throttle.

After year 2008 Dingell was stripped of his House Chairmanship and replaced with Barney Frank, who did away with the emergency passing power “loophole.”

Gearing of 2000 rpm at 80 mph,
or 40 mph per 1000 rpm
usually results in near optimum fuel economy with typical engines.

Turbo engines usually get about the same “peak” fuel economy as non-turbo engines,
but turbo engine stay near this peak over a broader range of rpms.
5 psi boost is about optimum level for fuel economy.

Not a swap. However, I daily drive a 2012 Challenger R/T Classic 6M with a 5.7L Hemi and a 3:91 gear. Mostly city diving and some highway each week. Averages 26.9 MPH, but I do tend to get the RPM's up between each shift. It has just over 72,000 on it now and has been flawless. Have only changed the tires and front brake pads once and regular oil changes. All else is original.

You sure that's not 16.9?
Your numbers are even better than the usually inflated EPA numbers, and with 3.91's and mostly city?
2012 is a heavy car too

The 5.7-liter V8 engine provides drivers an EPA-estimated 15 MPG in the city and 23 MPG highway

I agree. That is fantastic highway mileage for nearly any V8 engine.

Though, at one time my company car was a 2006 Ford Explorer, 5.0LV8, 6-speed automatic and 3:55 rear axle. Driving east from Eau Claire WI at 65 mph (tail wind) and about 1600 rpm it was showing 24 mpg on the fuel computer.

My 6.4 I built for my 3/4 ton can get close to that, a 5.7 in a much smaller vehicle should easily do that if done right. My old 2011 1500 with a 6.4 and MDS would hit 20 MPG after I put on a vararam.

A 46RE came from a hemi truck?

Shifting gears, literally. I found out none of the Crhysler RFE trans will mount/work with the built 5.2 Magnum I am planning for my 56 Dodge pickup build.

So, I'm going ZF on it. The ZF 8HP-70 8spd tranny can mount to the 5,2 with an adapter plate. And a controller is available to make it shift properly. So I can gain the fuel economy of an 8spd trans and get the reliability of the ZF trans. I'll spend a lot less on getting a built 5.2 Magnum into the 400hp/375 ft-lb torque range than to spend $4000 just to get the remanned 4.7HO long block, and still try to get a good intake/exhaust on it. For the less money I spend on the built 5.2 Magnum I can spend more bucks on getting a good used ZF trans.

Since I'm cutting up my 2001 Dak cab to retain the firewall and floor to go under my 56 Dodge pickup, I can modify the Dak floorpan to clear the ZF trans. Now, it would be kinda cool to rig up the old pushbutton trans control from the early 60s to operate the ZF trans, if possible.

The old push button shifter from the 50s & early 60s was only a 3 forward position shifter with a reverse and a neutral, didn't even have a park, the park was the E brake on the drum on the back of the trans. Hooking that dual cable shifter to the modern 8 speed trans could be a challenge in itself.

Might want to mount that modern dial shifter to the dash in place of the old pushbutton shifter.

On my 54, I had the radio, the head light switch, and a pair of cup holders built into the center glove compartment on a sliding platform. When the glove box was closed, everything was out of sight. When we drove around, it was open so we could use the cup holders built into the glove box door. Replacing the radio with the dial shifter could be fun. No one would know how to shift the truck unless you showed them, and everything inside that closed glove box door sure cleans up the dash. Unfortunately, I no longer have a picture of that glove box arrangement.

I knew I could get a response from you Gene. Yeah, the push button control was kind of wishful thinking. The dial shift control is a feature I am getting used to in my company car, its ok but I guess I'm old school and prefer a shift lever of some kind. Though not having a floor mounted shifter will keep the cab floor uncluttered. With the ZF 8spd automatic I'll have to get more used to a dial shift control. That means I have to plan the wiring from the dial control to the trans also, wherever I decide to locate the dial control. Locating the dial control like the old push button control was mounted would kind of make it a tribute to the old push button control. It would also keep the rest of the dash simple and clean.

My 56 has the center glove box door, and I have plans for that already. I measured the HVAC controls and factory stereo in my 2001 Dakota and they"ll both fit in the area behind the glove box door, with a bit of, er, adjusting and a custom inner panel (the stock center panel of my Dakota may fit with some changes). I do like the upright touch screen in the center of the dash in my 2021 company car, a Ford Escape. The touch screen, backup camera/screen is great, though it stands up so much it breaks up the flow lines of the dash.

So in my 56 I plan to flip the center door around so its hinged to the top, with a feature to keep the door flipped up when opened. When the door is flipped down and closed the dash will look clean and simple, just like a classic 56 Dodge, not showing the HVAC controls, stereo or GPS. When opened and flipped up, the Gamin GPS I mount to the inside of the door (upside down when the door is down) will be in a position similar to that in my 2021 Escape. Garmin also makes a back up camera license plate mount, that shows on the Garmin screen when the trans is in reverse. The back up camera is a great feature on new vehicles, and will help me line up to hooking up my trailer. Having a back up camera is a good feature for when my wife drives my 56 as she is used it in our other vehicle. I figure drink holders will be a feature of the center console with the seats I decide to use.

No reason why you could not modify the pushbutton controls from mechanical to electrical incorporating the dial shifter electrics.

Your post reminded my feeble old poor memory about using micro switches with the push button controller.
Thought it was a neat idea that I will never do, ... took some time but I found it.

Sorry I had a senior brain fart there. It’s a 545 RFE

Talked to Russel at Sound German Auto about the shifter control for a 8HP-70 tranmission. He said as long as the trans comes out of a Charger or Challenger (not a Ram) it can be controlled with a dial shifter or a floor shifter. The floor shifter allows for the manual shift option +D- , but the +D- control can also be done with many other types of switches.I asked about using steering wheel switches like those on the back side of a Jeep Liberty steering wheel. The operation would be similar to paddle shifters. So based on that logic the old dash mounted push button control could also work for the PRND trans functions.

I bet that would work to control the 8HP-70 trans, since it is essentially equal in function to the modern dial shift control. Cool.

if you go that route, please take pictures of what you do to the shifter.