optimum hwy RPM?

I'm not sure how some of y'all accept such low highway mpg from a reasonably stock 383? When our '66 Newport was in its earlier days, we made a trip to deep West TX to see relatives for a holiday weekend. Out past Odessa, Dad said "This thing feels like it wants to run 90", so he eased it up and we "cruised" as I checked out the long distance AM radio reception on that night. This was after we'd filled the tank, so when we filled it up before leaving, the mpg came out to be 15mpg average. Which tended to mirror what "Motor Trend" had with a '62 Newport 361 2bbl road test. That one had the BBD 2bbl, ours had the Stromberg. Later, keeping the speed down to about 70mph, it was more like 17mpg. Just a stock 383 2bbl 2.76/H78-14 deal. With E10, probably would be about 6% lower?

When I was driving the '70 Monaco all the time, I had a friend from college who had a '69 Super Bee with a Holley. 3.23 and G60-14 tires on the back, F70-14 on the front. He said he usually got 15mph on the highway. By comparison, my Monaco 383 "N" 4bbl AVS would do more like 16.5mpg. 3.23/H78-15.

Once, we got the carb on the '66 rebuilt at the local Chrysler dealer (where we bought it with 7100 miles when the new '67s came out). Suddenly, mpg dropped to 11mpg all the time. Why? The "famous" air cleaner air horn warpage, which didn't let any vacuum get to the power mixture piston to keep the power valve closed. So, it was on "full power mixture", about 12.5 to 1 AFR all the time. I made a spacer gasket to fix that problem. Mpg went back to normal. Overall town/highway driving mix we did back then was normally about 14mpg. That's what the cars would do when newer and "gas was gas", even with LEAD in it.

In those earlier days, when we'd get it tuned-up a the Chrysler dealer, they always did the final check with a SUNN Air/Fuel Ratio meter. Once the rpm went past about 2000rpm, in "P", the meter quickly swung the needle to 14.7:1, or "stoich", mixture ratio. At idle, it was usually in the high 12s range. All "normal", for back then.

Considering the availability of wide-band 02 sensors, getting the mixture at cruise can be monitored in real time. Just need to get the sensor installed into the down pipe(s). I know that people seem to like having "rich for power" mixtures for normal driving, but isn't that what the power valve is for? Leaner for cruise, richer as the manifold vacuum drops with more throttle input when more power is needed.

As for the engine sounding "happy", this seems to be easier to do with more cam than what the basic "standard cam" was on the 383s, I believe. This "happiness" can be felt with tight throttle response and ready power with the least throttle input. In one respect, it seemed that after I'd spent hours in the cars, on the open roads of western TX, just me and the car, I could start to understand and "tune in" on these things. Once recognized, it was easy to tell what was going on and when the car was having fun too. These are things that you probably won't notice on a 30 minute commute (and all of the related distractions). Getting these cars out and doing 300+ miles/tank, on open roads in the "country sides" rather than Interstates, you really come to appreciate how Chrysler did their engine tuning, chassis tuning, and designing cars for "The Great American Road". A real "cruise", rather than the 35mph boulevard "cruise" that GM designed for.

Just my experiences,
CBODY67
 
I run Mathilda, my 66 Newport w a 383, 2 bbl Stromberg, Sino-imitation distributor w single points, a 2.76 rear end and Champion 10 plugs and get ~14 mpg on I-10, 13 mpg on 4 lane paved streets w moderate traffic, and 9 mpg on shitty barrio streets pocked w more crater than paved surface area and crammed w pot-puffing, cell phone jabbering idiots. That 2.76 742 rear end is a blessing which I mean to keep for so long as I drive her. IFF blessed w some windfall, I hope to get a mid 60s Dodge 200 w the same power plant and tranny I run in Tilly, but a lower rear end for work.
 
I'm not sure how some of y'all accept such low highway mpg from a reasonably stock 383? When our '66 Newport was in its earlier days, we made a trip to deep West TX
....
These are things that you probably won't notice on a 30 minute commute (and all of the related distractions). Getting these cars out and doing 300+ miles/tank, on open roads in the "country sides" rather than Interstates, you really come to appreciate how Chrysler did their engine tuning, chassis tuning, and designing cars for "The Great American Road". A real "cruise", rather than the 35mph boulevard "cruise" that GM designed for.

Just my experiences,
CBODY67

Nothing like cruising Texas highways to know your car! DAMN! That brings back some cool memories!
 
Take a vacuum reading and it should be rock steady if you aren't moving the throttle. What ever that reading is will be pretty close to your MPG.
Just a mechanically challenged fly on the wall watching this interesting thread trying to learn a thing or two. Thanks Kevin for this useful rule of thumb.
 
as were not ******* mind readers and don't know WTF your looking for!

CRYSTAL.BALL.MISS.CLEO.jpg
 
Hey @swisherred , question for you. Did you get to drive you car before taking it apart? If so how did you like it then?
 
I was able to drive it some before taking it apart...it was crap...kinda. It needed a lot of work. Suspension was worn out and the engine struggled with very poor tuning in the carb and a lot of oil sludge. Acceleration was decent but it felt like I was blowing gas through it without getting the power from it. I have since changed replaced everything I can suspensionwise and refreshed the motor some..added new electronic ignition and replaced the worn out timing gears. Its much better. Now I'm hunting the final tuning/gear ratio and tire size for it to be most efficient/happy and run the best it can for what it is. I feel I need to rebuild or replace the carb...maybe tire size and dif ratio. Still feels like I'm not getting the power it should. No idea on mileage because I've never filled the tank. I'm trying to tune it for my driving habits. Back in the day the apeed limit was 55....cars were tuned for that...I drive 70 everywhere because the speed limit here is 65. I want it to be happy at 70 and not add too much heat to the trans or totally kill my mileage.
 
Back in the day the apeed limit was 55....cars were tuned for that...I drive 70 everywhere because the speed limit here is 65.

Not really. The speed limits were 65 about that time and had been that way for a while. It was the gas crisis in 1973 that brought the speed limits down in 1974 to 55MPH. These cars were built to cruise at 65-70.

My '70 Chrysler sees 75-80 MPH cruising speeds often... The trip to Carlisle includes the much hated PA Turnpike with 70 MPH speed limit. No issue... The convertible top raises about 2-3" of the center bow at 80. LOL
 
Not really. The speed limits were 65 about that time and had been that way for a while. It was the gas crisis in 1973 that brought the speed limits down in 1974 to 55MPH. These cars were built to cruise at 65-70.

My '70 Chrysler sees 75-80 MPH cruising speeds often... The trip to Carlisle includes the much hated PA Turnpike with 70 MPH speed limit. No issue... The convertible top raises about 2-3" of the center bow at 80. LOL

Yes. The gas crisis did cause an adjustment to the nationwide interstate speed limit. I think initially, for a short time, it was actually set at 50 mph, but nobody observed it. My '66 300s with 3.23 diffs get pretty happy at 70 MPH.
 
I want it to be happy at 70 and not add too much heat to the trans or totally kill my mileage.

If I was in your shoes I think I'd just pick my wheel/tire combo and run it with the stock gears in for a while. You might find that you like how it is stock and not have to spend that time/money for a gear swap. If you don't like it then it's a fairly simple swap. My car (413 and 2.7x or 2.9x gears) gets around just fine highway or town and I get around 15 mpg avg. If I was to change gears I'd go 3.23, but I really don't have any reason to do so right now.
 
383 has big block grunt and can scream like a small block..
68 Newport..383 750 Eddy carb Comp 268H cam. 2300 stall 2.94 open gears 28 inch tall tires 23570R15. Around 2500 RPM at 70 MPH. That is the Boab's sweet spot. I can lightly accelerate and move along quickly.
I installed a 3-2 part throttle kuckdown in the valve body so even at cruising speed I can still mash the gas and drop into second to pass. Second is good till 90!!!!!
2.94 gets overlooked. Great medium between highway and street gears. I get 15.5 MPG out of the BoaB when I behave on the intertate highways!!
As mentioned there are many factors involved to provide a perfect answer. Just wanted to share my set up and hope it helps.
 
Forgot to mention last night R15 P235/75 rear tires, R15 P215/70 front. I might lean down the back tires a little in another yr or 2. Yes, I too can roll in 2nd gear up to 90.

Overall, this old car does quite decent on the highways. Carb work and new timing should improve efficiency a bit. Just a matter of money and time.
 
Consider that a '65 413 Imperial had the 2.9_ rear axle ratio with 9.15x15 tires. A 413 New Yorker of the same model year had 2.76 and 8.55x14 tires (as did a similar 383 2bbl Newport) as standard equipment. Although the Imperial was heavier, it also had a taller tire and more gear. The New Yorker with the same 413 engine specs had a taller gear and less tire diameter. Think they weren't up to something in those combinations? To have highway cruising rpms in a particular rpm range?

With respect to the tire choice issue, I'd be more concerned with the load carrying capacity at 32psi being similar to what the OEM tires would do, THEN worry about the aspect ratio and such.

Just some thoughts,
CBODY67
 
Think they weren't up to something in those combinations? To have highway cruising rpms in a particular rpm range?
With respect to the tire choice issue, I'd be more concerned with the load carrying capacity at 32psi being similar to what the OEM tires would do, THEN worry about the aspect ratio and such.
Just some thoughts,
CBODY67

I won't argue with this at all, well said!
 
Consider that a '65 413 Imperial had the 2.9_ rear axle ratio with 9.15x15 tires. A 413 New Yorker of the same model year had 2.76 and 8.55x14 tires (as did a similar 383 2bbl Newport) as standard equipment. Although the Imperial was heavier, it also had a taller tire and more gear. The New Yorker with the same 413 engine specs had a taller gear and less tire diameter. Think they weren't up to something in those combinations? To have highway cruising rpms in a particular rpm range?
CBODY67
My '62 New Yorker had 2:92 gears with the 413; I got 18 MPG highway. I was ecstatic!! Wow, I can tune a car!!! Then I read in "Chrysler 1924-1900" that the '62 New Yorker won the MPG contest for "luxury cars" in '62 with... 18 MPG. Sheesh! It wasn't me! I loved that mileage though.

My parents '65 Newport 383 HP got 17. Don't know what the rear gears were.
 
Chrysler was a top contender in the Mobil Economy Run each year, for years. Especially in the "larger car" classes.

There was an article in one of the now-gone Mopar magazines about how Chrysler operatives went about their successes. Not unlike the way they dominated the super stock classes at the drag strip, too.

The drivers knew their cars "inside out" and how to make them perform as they needed them too. Like having an open drink to gauge the steepness of an incline by how the fluid tilted in the container, then knowing how much throttle to use to maintain speed without getting into the power mixture in the carb. And many other little things, too.

The article mentioned that a college kid was an observer as the new cars were broken-in, to ensure that nothing "illegal" was done. BUT when a Pontiac came through with 2-ply tires OEM, the driver went to an empty parking lot and did enough hard cornering to weat them severely, so that new replacements could be purchased (usually 4-ply tires, for lower rolling resistance) before the contest began. As the driver ground-down the tires, the college student observer sat under a tree and read a book, the article noted.

End result, the Chrysler drivers were probably much more prepared to "do battle" than many others were.

A later comment was that in the classes where Chrysler was super-competitive, they might lean the carb a bit on those models, or put a manual trans with a "highway" rear axle ratio as standard equipment. Which (in one case) allegedly resulted in them not being able to easily back over the curb for the winners' awards display. It was a big game that they usually won.

Looking at the results back then, it was amazing how far behind the Chrysler products the similar GM cars were. We all knew how efficient the TorqueFlite automatics were, as the mpg results seemed to verify that. Every now and then, a Buick or Olds might sneak into the winner's circle by a few tenths of a mpg.

CBODY67
 
I think this is a great question and way more relevant than asking about how to get 1/4 mile times down (apologies to MEV, and my favorite New Yorker).

I didn’t read all the responses (I’ll do that later when the kids are asleep), but engines get best fuel mileage at the torque peak. Do tuning for max mileage would mean set your gears for desired cruise speed at torque peak.

Of course, that’s not the question he asked. But I think it gets perspective. I think if you want responsiveness, having your revs near the efficiency point is a good staring place in the absence of data.

I don’t know about a 383, but the 440/350 has a torque peak of 2800 rpm. I would propose that if you are at the torque peak at 65, it would give really good throttle response in 3rd above that speed and would have some “headroom” in revs for the lockdown.
 
2500 is nice and comfy and will respond when you goose it. 3000 is the top end for most people to sit and listen to for an hour and you likely won't need to kick down to pass
 
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