Fuel mileage and related discussion

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Another thing if you do a lot of Hyw driving lower your car a couple of inches with the front 1.5 degrees lower than the back and a front air dam and cut the amount of air that enters the engine compartment so long as it don't create over heating problems.
 
I haven't driven my 273 enough yet to measure mpg, but daily drove my 65 Newport 4 dr w/ 383 for years. I got 18 mpg hwy, 13 mpg city. That is with Crane XR700 ignition and Holley Pro-jection TBI w/ O2 sensor. Don't know the gearing, but the motor seems to turn slow at 65 mph (no tach). That car is ~4000 lb, so I hope to do much better in my little Dart, and better that most here. I put in a fairly mild cam (0.422/0.444 lift) w/ Rhoads leak-down lifters. The later should help mileage.

Many ways to hyper-mile. Most are obvious to anyone who bicycles. Every time you use the brakes, you waste energy. The old 70's recommendation to press the accelerator lightly like an egg is under it doesn't match engineering theory. An engine is more efficient at higher throttle, as long as you don't go so far to open the power enrichment valve or squirt the accel pump. Of course, if you accelerate fast from a stop light only to brake hard at the next, that is stupid. In road trips in my T&C 3.8L, I try to get behind a big truck. It has an instant mileage display, which is fairly accurate. I get 23 mpg alone and 33 mpg following a truck at 60 mph. At that speed, I can follow at a safe distance, though the scenery never changes. Most people just must go faster, which is fine but don't whine about poor mileage.
 
So you were turning about 3360. Balancing the wind resistance load with final drive ratio with optimum engine efficiency, was your best gas mileage.
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What is the answer to your ponderous pontification? :)

rumblefish360 said:
My last configuration of my '79 Magnum was a (Dead on arrival) '1978/400 cid engine/727/3.55 on a 245/60/15. Best mileage on the Hwy. was 17 @ 70-75 mph. Any speed lower than 70 mph consumed more gasoline. RPM's unknown, no tach.

Now you all think about that one for a bit and work it out in your head.
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So, in other words pumping more air with out more fuel will allow a more efficient VE or something? Or simply due to more oxygen availability?

Is this why modern cars just stay in high gear when the cruise is on, up a hill and they open the throttle valve considerably more rather than downshifting a notch?


BillGrissom said:
An engine is more efficient at higher throttle, as long as you don't go so far to open the power enrichment valve or squirt the accel pump.
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Sorry about the hijack, I got excited here.
 
Rice Nuker said:
So you were turning about 3360. Balancing the wind resistance load with final drive ratio with optimum engine efficiency, was your best gas mileage.
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What is the answer to your ponderous pontification? :)
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I'm not sure of the question.

Your asking me about balancing the wind load against the final drive ratio when my engine was running at it's best?

Help me out here a second. How do you balance wind load?
How do you balance it against the drive ratio?
The engine was a bone stock 1978/400 minus the headers and 2-1/2 H piped Hooker Aero Chamber mufflers and a Bolley Street Dominator. The TQ was a small primary, electric choke version.

(The larger primary was better for performance on the primary side. Nothing of note on the secondary side between the two carbs.)

Anyways, absolute best notes mileage was 18mpgs Hwy. an amount not always obtained. Best mileage obtained with help, AKA behind a tractor trailer was 22.
Around town was approx. 10.
 
Rumblefish: Here is your info you posted:

My last configuration of my '79 Magnum was a (Dead on arrival) '1978/400 cid engine/727/3.55 on a 245/60/15. Best mileage on the Hwy. was 17 @ 70-75 mph. Any speed lower than 70 mph consumed more gasoline. RPM's unknown, no tach.
Now you all think about that one for a bit and work it out in your head.


:) It seemed you were posing a bit of calculation question to see who could figure out why your vehicle got such good gas mileage at such high rpm. So, I was taking a stab at it.

Let me rephrase my answer slightly: The sweet spot between mechanical advantage of final drive ratio verses the diminishing return of rate traveled over wind resistance while maintaining in the optimum torque rpm area of the engine design yielded the best mpg at 72 mph at 3600 rpm or so. I think a similar process was already posted in this thread.

To balance or tally up the wind load sweet spot in concert with the other variables would be something I can only crudely speculate on since I dont understand the math or the process to come up with a logical final drive ratio or matching power curve for the engine.
 
Here are some rudimentary findings on the "peak torque RPM vs peak MPG RPM" subject.

Using the EVIC (manually verified to within .5 mile per tank) in my '00 Dakota 5.9...

Spec (what I can find anyway) says TQ peak is 345 @ 3200 RPM

3.92 gear, and .69 factory O/D, for final drive of 2.7:1
Cruising on level roads-

50 MPH - 1700 RPM - 19 MPG (real time)
60 MPH - 2000 RPM - 16.5 MPG (lots of switching between 16 and 17)

Next time I have the opportunity to get up to 3200 RPM, I'll record the speed and mileage, but it'll probably be a while, and I kinda suspect it might be single digits.

* caveat- Until the O/D kicks in, I have the benefit of the 3.9 gear, so I'm not fighting a 2.x ratio to get up to speed.
 
Carb tune is pretty important. Yeah maybe the car works fine with the tune it has but, It could be running at a 11 to 1 fuel air ratio at cruise. I think anyone considering getting good mpgs should get a Wide band o2 sensor and a jet kit for their carb. wideband will take the guesswork out of carb tuning and you can record your readings and/or see them in real time.


Also here is a neat MPG experiment someone did:
http://board.moparts.org/ubbthreads/showflat.php?Cat=0&Board=best&Number=3944014&page=0&fpart=all
 
If it's got a really good tune up on it, should be able to pull down 20mpg or more on the highway.

City driving is very dependent on how heavy your foot is.
 
I did 50 MPH with the O/D off on my way home.

2450 RPM and 12 MPG.

After three months in my bone stock 318-2 73 Sat (unknown rear but likely 2.76), I'm up to averaging about 13.5 MPG in mixed driving (same commute as Dak).


Edit- average for Dakota 5.9 in mixed driving is 14.1

4200 pounds, so it's like a C body, or a late B body with at least 2 passengers.
 
I have a ton of suggestions to improve your fuel economy, some of which have already been provided here. Here's my list of things that haven't been mentioned yet:

1. Take some friction out of the cam and valvetrain. Going with a full roller set up will definitely help. If the budget is tight then just swap the stock rockers for roller rockers and you'll be rockin'.

2. Get rid of that damn old school fan. I removed my direct mount fan (i.e. no fan clutch) in place of an electric piece and my highway milage instantly went from 15 to 17. Plus my highway ride got a lot quieter, with much less whooshing noise coming from the engine compartment.

3. Put a good synthetic fluid in, well, everything. This is the same theory as number 1 - reduce friction. I'm not sure how much impact it would have in the automatic transmission, but in my 8.73 / 3.23 sure grip I went from standard gear oil to some Royal Purple, and my milage got noticably better (unfortunately I didn't measure it).
 
I was in college in Daytona Beach, FL during the 1973 OPEC gas shortage. I had to drive home for summer 1974 and was unsure whether getting gas on I-95 would be a problem, especially in parts of North and South Carolina where the interstate was not complete. In Daytona, some gas stations were closing early, 5-8 p.m. instead of staying open until 10 or 11 p.m. The last thing I wanted was to sit on the side of the road until gas stations opened in the morning.

I was driving my bone stock '68 GTX 440-4; 3.23 rear; A-727. The previous owner (original) had 15" chrome slots on the rear with Mickey Thompson G 60-15s (they wore down and I replaced them with G (or F, I can't remember) 70-15s for interstate driving. I adjusted the AVS to open the secondaries at about 65 m.p.h. The government, in their infinite wisdom, had decreased interstate speed limits down to 55 m.p.h. I planned on traveling at 60 m.p.h. (roughly 2250 r.p.m.) so as to better compute my progress (1 mile/minute) in getting to the next town in case I needed gas.

The bottom line - the GTX, loaded with me and everything I owned, clothes, books, and tools - 22 m.p.g. on the flats of Georgia and the Carolinas. The engine was just humming along - right in the sweet spot. It decreased to 18-20 m.p.g. once I got into the rolling terrain in Virginia and Maryland.

I never computed around-town driving because it really was not of concern at that time.
 
BadAss71 said:
With my 360 magnum and overdrive trans I have been averaging right at 24 city and highway combined. This is with a 2.94 gear and 26" tall tire. At 75 mph the motor is turning 2000 rpm. I can pull hills easily without the trans coming out of overdrive at that speed, with an occasional unlocking of the converter. On my road trip to Denver I averaged 26 while cruising 75-85 mph. I am not complaining one bit!
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are you using the fuel injection or did you convert it to carb?
 
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