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.