Fuel mileage and related discussion

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Furious65

Furious65

C-body Guy
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What I can expect roughly for MPG from my '70 Duster with bone stock 318-2bbl?

Also, what is considered the ideal cruise RPM in our cars to get the best fuel mileage on the highway? I have been reading about OD's and other MPG related topics trying to figure out how to maximize fuel mileage under all conditions. I was reading a thread at Moparts (Fifth Ave exhaust thread) where the 5th's were getting some decent MPG considering their weight all running super high rear gearing (2.24-2.45) yet it hurt the in town mileage due to not enough gear to move the car easily from a stop.

Just thinking out loud, something like an 42rh/46rh trans, running 3.23 rear gears with 26" rear tires, should give a cruise RPM of around 2160 at 75mph. A 2.74 rear, everything else the same, would yield a 1832rpm at the same speed. I would think the latter would be better overall for in town and highway mileage.

Curious what your opinions are on how to maximize the overall MPG's from our cars and what the ideal cruise RPM should be!

Plus considerations. What about camshaft selection and whether increasing compression would help or hurt MPG?

Sorry, this thread really should be in "General discussion". My mistake! Could a mod possibly move it?
 
My 65 Dart 273/904 was getting around 11.5 around town with a 2 barrel and 2.93 gears.

I replaced the mopar electronic ignition (orange box) with a chinese all in one distributor with HEI, flamethrower low resistance coil, eliminated the ballast resistor, gapped the plugs to .050" and advanced the timing from 10 to 15 degrees btdc and started averaging 13.

I switched to a stock four barrel setup (500 CFM Carter AFB) and lost about 1/2 MPG or about 12.5.

The very best tank, with the 4 barrel, and all freeway miles and keeping the speed down got me 15.5.

I think the car should do better considering how light it is. My compression is down around 120 PSI and one cylinder has a leaky exhaust valve and only measures 90 PSI.

I think the engine would be more efficient with a higher compression ratio, provided it's not so high that it causes detonation. A bigger cam would probably hurt unless the car is geared low and having trouble breathing at cruising speed.

Overdrive would be an obvious remedy and I would probably do it if it didn't involve cutting the crossmember and modifying the hump. A lock-up torque converter would also help. Of course a five speed conversion would be the best.
 
Ideal cruise rpm is a variable with a few considerations to think about. While one can get good mileage here (sea shore) another may not with the exact same set up where they live. Places like Co. At a mile high have less Ir and the game changes. Also, places with more hills will see less mileage.

I had a '79 Dodge Magnum (think Cordoba) w/a 360, Edel.-600 CFM on a factory iron intake, dual exhaust off the manifolds w/twin cats after the H-pipe, 2.76 gears on 235/60/15's. A MSD triggered the vacuum advance distributor to net me 20 MPG's on level land.

More could have been done. IDK about the return on the investment with a camshaft change and electric water pump, etc......
 
Its RPM, the slower the engine turns, the better the MPG..now yeah, 700 rpm at 70 mph isn't going to work but 2,000 rpm will, mine turns around 4,000 at 70 mph, guess what, the MPG isn't so great, lol
 
dodge freak said:
Its RPM, the slower the engine turns, the better the MPG..now yeah, 700 rpm at 70 mph isn't going to work but 2,000 rpm will, mine turns around 4,000 at 70 mph, guess what, the MPG isn't so great, lol
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yep......couple usable low rpm with a lean a/f ratio and as much timing as it can stand and it will help. Pretty much start from scratch, just like any "good" build it is more about the package than any individual part...
 
In my 66 dart i have an 87 gran fury highway patrol 318 engine. It has 360 heads, intake, and exhaust manifolds, factory everything, camshaft, windage tray, springs, valves, heads. MSD analog 6A box, NGK V-power plugs gapped at .050" and feeding it is a 4175 series holley spread bore. I cleaned up the valves and freshened up the motor a bit. The transmission is the original 904 with lockup, which is an aftermarket pro-king 1800 stall converter. the rear end is 3.23s, with the poverty 13 inch steelie rims on all 4 corners. doing 65 its sitting at 3000-3100 rpms. on a 520 mile roadtrip (all in one night) from missoula to spokane i averaged 18.4 mpg at 16 in/lbs of manifold vacuum almost the entire time on the freeway.
 
many things to consider here but everything mechanical has a sweet spot where you get the most for the least..even the speed you paddle a canoe.
your maximum lo speed torque must be at your cruise speed for maximum efficiency.
 
66 barracuda, 360/727/2.76 (8.75 rear end) 4bbl, lunati 60403, 9.5 static compression high 11s to low 12s in city, 14.5-15 on freeway I feel like I could do a lot better...
 
My Dart currently has a 318, mechanical cam similar to a 340 stick cam, RPM performer, and 625CFM (or so) AFB. 3.89 Ferd gears, tall rear tires. 70 mph is about 32--3400 All my driving is short hops and I'm not very careful. I have yet to get below 12, and get 14-15 routinely. Again, this is short trips. Longest "highway drive" is typically 45 mi one way and some of that is on lower speed surface streets.

You CAN get too low on RPM. 2K at 70 just might be a little low for a short stroke engine. If you get "under the torque curve" the engine may be lugging a little at such low RPM.
 
67Dart273 said:
My Dart currently has a 318, mechanical cam similar to a 340 stick cam, RPM performer, and 625CFM (or so) AFB. 3.89 Ferd gears, tall rear tires. 70 mph is about 32--3400 All my driving is short hops and I'm not very careful. I have yet to get below 12, and get 14-15 routinely. Again, this is short trips. Longest "highway drive" is typically 45 mi one way and some of that is on lower speed surface streets.

You CAN get too low on RPM. 2K at 70 just might be a little low for a short stroke engine. If you get "under the torque curve" the engine may be lugging a little at such low RPM.
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Thanks gentlemen. This is the kind of info I'm looking for. Trying to find a good balance to get maximum mileage is tricky.

FYI, when I was talking about a possible cam swap I was thinking something very small like the Crane 693971 or MP P4452757 (something that will pull at least 15in-hg). Basically a more modern cam profile that is somewhat close to stock to help get excellent torque around 1800-2000 rpm. I know my Dakota (4.7 w/auto OD) would turn about 2250 at 75mph and pulled that with no problem. I figure since the 4.7 and 318 are very close in stroke length, 3.31 vs 3.4, and the fact the Dak weighs in at about 4,300 lbs (about 1k more than a Duster!) and is as aerodynamic as a brick, the little 318 should be able to pull 2k at cruise with the right selection of parts.

Lastly, reading over at Valiant.org (http://www.valiant.org/duster.html), they show the 1976 Feather Duster as pulling some serious mpg. They list the auto at 22/31 city/highway and the manual at 24/36! Yes, it was lighter than a standard Duster, they state 180lbs to be exact, but that really isn't that much but it helps. Main reasons cited for the mileage increase was due to a 1bbl carb, economy curved ignition, and a large free flowing exhaust. I can tell you this, if I could get those numbers I would be extremely happy! There should be a way to get similar mileage, and with the more modern technology, retain some of the stock performance of the 1970 engine. Thoughts?
 
My '70 Duster has a 360 with Magnum heads, 10.5:1 comp, Voodoo 256/262 cam, shorty headers, Mallory CD ignition setup, Carter AFB 625 cfm carb and 904 trans with cheap (slippy) 2500-stall converter and (gasp!) stock 2.76 gears and it averages 14 MPG with mostly city driving and my lead foot. A long highway trip got me around 17 MPG but the converter slippage is the biggest killer; if I had a manual trans it would push 20+ no sweat I'm sure. BTW it runs great on mid-grade (yes that's 87 octane here at mile-high elevation) as long as I don't floor it in 3rd at slow speed or hammer it in 100* weather. Also from what I've noticed running this cam in a bigger high-compression motor is the MASSIVE low-end and mid-range torque; it currently cruises around 2800 RPM at 75 MPH and I think it would have no problem running that speed at 2000 RPM instead.

However the best setup for mileage in my mind would be a 318 with zero-deck pistons, closed-chamber heads (Magnum or aftermarket) for around 10:1 compression, similar cam to mine (low duration, high lift), headers, and good spread-bore dual plane intake to mount a ThermoQuad carb. All hooked to an overdrive manual (4- or 5-speed depending on your $$$$) and 3.23 gears. I think that kind of setup in a 3300-lb Duster/Demon (better aero than the 'rectangle cars') would push over 25 MPG on the highway and 325+ crank HP to boot.
 
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!
 
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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The 360 does have a longer stroke but I bet the 318 could pull 2000 rpm with the right setup. Curious as to what cam you running?
 
64 Signet, 360 hyd. roller cam, 650 edelbrock carburetor, 79 stock manifold, HEI built in coil, manual valve body 727, 3:55 sure grip > 19mpg and that's a fact
 
ir3333 said:
many things to consider here but everything mechanical has a sweet spot where you get the most for the least..even the speed you paddle a canoe.
your maximum lo speed torque must be at your cruise speed for maximum efficiency.
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BINGO!!!!!!

Slower engine speeds does not mean better mileage!

Things that help mileage are things that make the engine run better. More effectively. The easier it is to breath in and exhale will improve not only power but mileage. When it comes to a cam profile, look to a split duration cam since the exhaust ports are less efficient. After that, look towards your ignition. Getting all the fuel to burn is also a key item.
 
Fuel efficiency typically follows the torque curve of the engine. The torque curve of a typical engine starts low, builds, flattens out then drops again at high rpm. I find most economy at the low end of the flat spot.

I get between 18 and 23 mpg on 66 237 Barracuda. While it is fairly flat here there are a huge number of stop lights. Interstate speeds are 70 MPH, and the traffic is often above that. I get slightly better mileage on the highway. I drive briskly, and in normal driving easily pull away from the modern cars when starting. While feathering the gas to maintain speed, helps it is also important to get up to speed quickly, and run the engine in its efficient area to get there.

Proper idle speed, mixture and timing are important. I typically adjust the idle fuel mixture to curb speed 30 to 50 RPM below max by going slightly lean. Too lean results in misfire and poor economy.

Disk brakes do not have return springs, so they drag more. Tire pressure and alignment are important.

The alcohol in gas leads to lower mileage. Back when there was real gas, 25 mpg was typical for me.
 
First thing you got think of is it cost effective if say you get 10mpg now and spend $400 a month on gas now if you increase your gas mileage to 15 mpg you'll now be paying $300 a month in gas which is a $1200 dollar savings a year but if you spent $2400 dollars to get a 50% increase might not be worth it so the first thing what's the budget then you can decide what gives you the most bang for the buck. Second do you want gas mileage while increasing, maintaining or decreasing performance?

I don't agree that cruising rpm got to be at peak torque to get best gas mileage, it's true an engine most efficient at peak torque, the more rpm your engine turns the more fuel it will use. Every car take a certain amount of rwhp to propel at a certain speed, at speeds under 50 mph it take less than 30hp but over 50 it increases rapidly 70 mph probably 70-80hp so as long as your engine produces enough hp to maintain speed you should be fine. Don't forget about driving habits my dad gets a few more mpg than me in my Jeep.
 
Plus I'd get David Vizards book Performance with Economy
 
I totally agree that you need to set your gearing on trans, rear end and tire diameter to operate your engine at the low side approaching peak torque during your typical cruise speed, taking into account your typical terrain. Because if you have lots of hills you will probably have to get out of high (1:1) gear (older 3 speed auto) if you are running 2.76 rear ratio or taller. I didn't because my engine made about 500 pounds of torque w/ 462 cubes.

I had 2.41 gears or 2.29, cant remember, but my car ran the smoothest and got the best mileage about 80 mph which was about 2500 rpm. I had a mild cam, mild intake mild exhaust and stock converter 3 speed auto, non lockup.

Anything you can do to reduce slip on TC and trans or run a manual trans is optimal. Automatic 4 speed overdrive with like 3.55 rear gears or a 4 or 5 speed 1:1 with tall gears is optimal because you dont have to step so far down to get a little power band to pull hills and you have a low starting gear ratio to eliminate start off load and getting in and out of the optimum torque area of the engine during acceleration. On an older 3 speed automatic I would run a low stall rpm and build an engine with as broad flat torque curve as possible. It is all a balance. That is why 5 speeds or lock up TC with overdrive are popular cause you can have a little higher stall converter and still lock the slush box and run tall gear when cruising. Plus kicking out of overdrive on a hill at .67 down to 1:1 is not a drastic jump which doesn't kick you out of your torque band on your engine to pull a hill.
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I believe the camshaft plays a good role, my engine seems to drink gas no matter how I drive it. The best it gets is 12-13 mpg

The speedo gear needs to be right on, if the speedo reads a few miles per hour higher then you really are going, it log in more miles then the car really went. Why is best to check mileage with those freeway mile posts, not what your odometer says. If I were to sell my car, I could easily put in a speedo pinion 1 or 2 teeth smaller then what it should be. That was an old trick we knew back in high school, take the speedo pinion gear out and replace it with a smaller-less tooth gear. Then the buyers think the car is faster and gets better mpg then it really does and the buyer is all happy and loves his new ride, ha ha ha
 
My dang 01 4.7 4x4 dakota gets 16 on a freeway road trip driving conservatively. It runs very smooth and clean with zero issues. I run 89 oct. No logic there that I can find.

I use a gps speedometer app on my Iphone that tracks trips. Eliminate speedo error. My speedo is 2.5 mph slow at 65 mph.
 
273

I don't agree that cruising rpm got to be at peak torque to get best gas mileage, I don't agree that cruising rpm got to be at peak torque to get best gas mileage, it's true an engine most efficient at peak torque, the more rpm your engine turns the more fuel it will use. the more rpm your engine turns the more fuel it will use.
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At or slightly below the best mileage will occur. This is fact. Proven. Your line- "it's true an engine most efficient at peak torque" is the key. This area where fuel is being used best to propel the car. Now, lets say it is at 2400 rpm. IF the car can obtain a Hwy. speed at this RPM, then the best mileage will be obtained.

Now the catch 22.

IF your Hwy. speed demands more RPM, the volumetric efficiency can rise allowing more MPG's because the velocity/distance out weighs the fuel being consumed and the car is propelled further than it would be at peak torque.

For the funny guys in the crowd, because there's always a fart in the breeze, your answer is no. You wont get best mileage at 7,000 where peak volumetric efficiency is at. Besides, your big roller cam drinks like a wino outta weekend lock up.

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.
 
You have to put the engine torque where it most responsive where you drive at.

Here is my observation from driving every day to work and back for the last 15 years.
1971 d100 slant 3 on the tree and a 3.91 rear 16 mpg
03 Jeep Wrangler 4.0 5-speed, 3.07 rear 17.5 mpg.
03 DRZ400 street and trail bike 73.5 mpg tank after tank 73.5 mpg RIP
08 Buell Ulleysses 54 mpg
 
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