Miles per gallon?

Slant 6 Engines

  1. kyler

    kyler Active Member

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    Hello,

    I have a 1966 Valiant 200 with a slant-6 225, 1 barrel carb, and 3-speed automatic transmission. What kind of mileage should I be getting? When I bought the car, I was under the impression it would be somewhere around 18-22 for an average. As it is, I have been calculating 10-11. Why am I getting such poor mileage? It just doesn't make sense to me.
     
  2. RustyRatRod

    RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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    Probably out of tune, or you're calculating it wrong.
     
  3. Dodge72

    Dodge72 Odd one out

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    I personally get 17 mixed MPG with a two barrel carb and 3.23 gears on my slant. With a one barrel carb and I assume stock gears you ought to be getting a lot higher than 10-11. Have you done a tune up recently? Valve lash set? Is the carburetor mixture set properly? Maybe timing is off? Lotsa little things can help or hurt.
     
  4. kyler

    kyler Active Member

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    Well, we thought that it was being caused by a bad thermostat. The car was never getting any warmer than slightly above the "C." After replacing the thermostat, the car gets much warmer and operates at a reasonable temperature (as a bonus, I am not freezing as I drive.). Here is the breakdown in case my calculations are off: 23217 at top off #1, drive for a few days and re-fill at 23343. 23353-23217=136. I put in 10.5 gallons, 136/10.5=12.95. (I did just notice I was a few miles off in my calculations earlier, but 12.95 still seems low.) I took a picture of the odometer at each fill so I know that to be accurate.

    My father is a mechanic by trade and says the carb appears to be in tune, not to mention the fact the car flawlessly. It has pretty old tires on it, but I can't think of what else could be causing such poor fuel economy.

    Also, that is the third top-off/calculation I have done. The other two came out at 10.5 and 11.2 mpg.

    Aside from a carb-tuning and valve lashing (which I am not all that familiar with, but I do know the cars run poorly when they need it.)

    What else could be causing this?
     
  5. bighammer

    bighammer Well-Known Member

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    Release the E-brake
    Air up your tires
    No jackrabbit starts
    Keep your speed down
    Give her a tune
     
  6. 67Dart273

    67Dart273 Well-Known Member

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    How do you know the speedo is correct? consider using a GPS. Most people have one, now, in some form
     
  7. Ddaddy

    Ddaddy I'm changing the World... one pixel at a time! FABO Gold Member

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    These cars never got 18-22 mpg, even when new. 15 mpg on the high end and 12 is normal average in mixed driving. 10 mpg in the city is common. None of the more than dozen 225 slants I've owned ever got better than 15mpg, and that was only after looong trips at steady speeds.

    I think you are trying to check your mileage too frequently. Average it out over about 500 hundred miles to see what you are really getting. Too much error in a 130 mile trip.
     
  8. RPMagoo

    RPMagoo Just An Old Motorhead

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    Enjoy it, and don't worry about the MPG. -- It's a hobby. -- Have fun.
     
  9. 67Dart273

    67Dart273 Well-Known Member

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    You don't mind if I find that a bit tough to believe? Maybe in the 73/ later smoggers, I would think the earlier cars would do better

    Let's have some perspective.

    My old 70 RR originally a 390hp, was swapped to a 71 junkard 340 in about 73. So we are talking here about a B body weighing close to 3900 with gas, tools, me, etc. ready to drive. This had a Dana 3.54, ran 3000 RPM at 70, a 4 speed. That 340 on the highway would get an honest 17.5 MPG

    My friend when I got out of the Navy in 74 and a nearly new Dodge 1/2T 4x4 with a 360. that thing empty would get real close to 14. We are talking here, a bout a nearly 5K lb pickup with whatever the "big" Armstrong tires were back then, 33" tall or so, and 3.55 gear.

    Of course in this case, there is a LOT of "headspace."

    What condition is the engine, compression, etc?

    Is the carb clean and works right?

    Is the distributor advancing correctly, including the vacuum advance?

    Just what conditions is the OP getting this mileage? You need a sustained, clear highway cruise to average this, a "long day trip." Around town? I agree, much lower. I would thing you could do 13-14 around town, depending on how you drive, and THAT is a very loaded issue.

    Tire size and gearing, and speedometer accuracy is a no brainer. That is, they MUST be accurate.
     
  10. pishta

    pishta I know I'm right....

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    Its amazing how fast a out of tune carb can go through gas. I had a Predator (when they first came out, not street version) on a 351C 2bbl (offy 4bbl, Erson Hi-Flo 2 cam, headers, 3.2x gears, C5) in a 57 Fordor. Yeah it was a pretty big car but it was also pretty light for its size, about 3500 lbs. I kid you not, I got about 5 mpg in that car. I drove it to my sisters house 15 city miles away and burned an honest 3 gallons. I knew because I filled up prior and she filled it again as a favor. Those carbs were race only, and lacked any kind of closed throttle idle circuit, and of course ran fatter than Oprah. The Feather Duster/Dart light were advertised to get up to 36 mpg. and thats only 150 lbs lighter than a normal Duster. Tuned carb, distributor and exhaust. I think everyone here would agree that the 2bbl version will yield you better mileage. And of course a diet will help too., If you got buckets in there, those weigh about 60 lbs a piece!
     
  11. Slantsix64

    Slantsix64 Well-Known Member

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    Check your compression slant six will run smooth at idle and will still have low compression cant get good mileage with a tired motor.

    also if your valiant has 13inch rims try putting some 14s helps with stopping power and drive ablity

    check point gap or just convert to electronic ignition

    plugs cables etc

    run a 180 or 195 thermostat

    make sure your fuel is not flowing threw dirty lines and a dirty tank

    i ran a holley 2280 two barrel with a super six set up ran great and had good mileage

    but number one check your comp it will show where your motor stands
     
  12. GaryS

    GaryS Well-Known Member

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    I owned only one slant six that got less than 20 in mixed driving with reasonable driving habits, and it was a new 1975, 225 Brougham fully loaded with options. It was in the15 mpg range. My current '64 Valiant convertible with a Super Six averages 17-18, with a high of 21.

    The biggest problem today is government mandated fuel running in old technology cars. Ethanol supplemented fuel will drop your mileage a good 10%.
     
  13. slickchick

    slickchick pedal cars only

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    I concur !!!!
     
  14. jbc426

    jbc426 Well-Known Member

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    My Slant 6 with a 904 and 2.96 something rear end gears was never in the upper teens early 20's around town either. I averaged 13-14 mpg around town, and got a solid 17 to 18 on the highway at 65-70 mph. It only dropped a half a mile a gallon or so once I had the motor rebuilt and Super Sixed by Doug Dutra, but both sets of mileage numbers were possible only if I stayed out of it pre and post rebuild.

    I swapped it out for a fresh 480hp EFI'd 408" 5.9 Magnum with a 4 speed overdrive automatic with lock-up torque converter and 3.91's in it now. It gets the same mileage now, if not better than it ever has. I'll do some more thorough longer distance mileage calculating this upcoming driving season. Same situation though, only if I stay mostly out of it.
     
  15. AJ/FormS

    AJ/FormS The end is near FABO Gold Member

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    In no particular order,and some/most already mentioned;
    SpeedO error
    Insufficient ignition advance,faulty vacuum advance,too-tight valves,low compression,fat carb
    bad convertor,restrictive exhaust
    engine running too cold;cold fluids and greases, ie winter running
    flat tires,dragging brakes,gears
    long warm-ups,short trips,stop and go,high speeds
    driving style,hard into it,
    winter
     
  16. barbee6043

    barbee6043 barbee 6043 FABO Gold Member

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    I certainly have to disagree. now if the slant is wore ou!! t, then yes poor mileage. should get 20 and better if decent compression and tune. of corse, makes a difference if stop and go city driving, I drive rural roads period. . I have 3 slant cars now and have had 4-5 others.

    people argue power of the slant. if good engine, they will cruise down the highway 80 mph and more. if wore out, then again it will be more of a dog.

    valve lash can make a difference too. easy to do. get it done. do a compression test.

    and by the way , the last 2 slant dusters,i had. autos ,one was a feather duster. both got 20-25 going down the backroads 55 mph rolling hills. best I ever had was a duster with the 4 speed OD, don't know what the rear gear was, got 30 on the 55 mph roads. efrsh stock rebuild.... the trans made the difference plus a GOOD engine! NO chi--.
     
  17. Charrlie_S

    Charrlie_S Well-Known Member

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    As others have said. Check compression, and valve lash. Then check and make sure the vacuum advance on the distributer is working. Check to make sure there is no smell of fuel in the oil (leaking fuel pump), and PCV system is working.
    PS: Make sure the timing marks are accurate. The dampers are know to slip after all these years.
    I get upper 20's on my 64 valiant, but it was rebuilt for economy.
     
  18. 66fs

    66fs Well-Known Member

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    Something is wrong.

    1. Either you do not know how to drive for mileage or something is wrong with the tune. Just enough pedal to maintain speed. Hold the pedal steady, do not accelerate or decelerate unless needed. The accelerator pump is just pumping fuel into the carb. Try getting and using a vacuum gage, the higher the vacuum the better the mileage.

    2. Engine not tuned for optimum power. Adjust the valve lash. If original, replace the points or adjust the dwell to specs. Replace the cap, rotor, platinum spark plugs, and get good wires (I use Taylor Spiro Pro). Check the vacuum advance to make sure it holds a vacuum. Adjust timing. Start at factory specs 2.5 degrees before Top Dead Center. Take a trial drive, up hills at part and full throttle. Keep bumping it up, 5* BTDC next, till it feels the best or pings. If it pings back off a step.

    3. It is hard to get a constant mpg around town. Constant cruising at highway speed with little change in acceleration is where you get the big numbers. My best slant got over 30 mpg. That was highway, and that 170 was not stock.
     
  19. nm9stheham

    nm9stheham Well-Known Member

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    Nonsense. My family owned 5 A-F body 225's when I was growing up, all bought new, and the '68 with auto trans got 18-22 mpg on the interstates, the 2 3 speed manual Sports were 20-23 mpg, the '76 Dart Lite OD manual got an honest 28 mpg and the last one, an '80 Aspen auto trans Super 6, got 24 mpg on the interstate. The in-town mpg was less of course, and a lot of cold winter starts will do that too, but not ever down in the 12-15 mpg range.

    For the OP, I would first suspect that the choke is misadjusted to be 'on' more than it should. Also, the manifold heat riser may well be stuck closed and preventing proper heating of the intake. And then there is ignition timing, ignition performance, and a worn timing chain will hurt performance and mileage, by causing the cam to be retarded.

    Some things that I would be working on:

    CHOKE: With the car cold, work the throttle once and see if the choke plate snaps fully closed. After the car has run a few seconds look down the carb throat, and see that the choke plate is open about 1/8 of an inch. Afters about 5 minutes, see if the choek plate is open. If not, pull out the choke mechanism and look for a small nut and screwdriver adjustment with the markings 'R' and 'L' for rich and lean. Adjust a bit to L to see if the car is helped.

    FLOAT LEVEL: This is very important to economy and driveability. Needs to be doublechecked.

    HEAT RISER: This is a small rectangular flapper on the front of the exhaust right below the carb. (On the later engines, this is a round disc looking weight.) This should move freely at any time and temp. If not, use some penetrant and patience to slowly free this up; there is a penetrant/lube at the Mopar dealer for this. This should be closed when cold and as it heats, the bimetallic spring should move it open about 90 degrees.

    IGNITION TIMING: Should be at least 5 advanced and some more is good. Suck on the hose to the vacuum advance and see if the timing moves more advanced. Check the points; the open gap should be around .018".

    IGNITION PERFORMANCE: This is a long topic, more than I care to write up here, and it has all been said in other threads. One easy thing to check is the resistance of the ballast. The stock ballast shoudl be 0.5-0.6 ohms cold and < 2 ohms hot. Any thing higher than 1 ohm cold needs to be replaced as it will compromise the spark energy.

    TIMING CHAIN: Very common to be worn and retard the cam 4-5 degrees. This needs to be examined with the timing cover off.

    LEAKING GAS TANK: Self explanatory!

    LEAKING FUEL PUMP: Smell the engine oil and if it smells anything like gas, put on a new pump, and change the oil IMMEDIATELY before driving again.
     
  20. moper

    moper FABO Gold Member FABO Gold Member

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    Something's not right. A 496 I built two years ago gets 15 in the highway...
    Check tire pressure, alignment (by a guy that knows torsion bar front ends), timing curve and vacuum advance. I also tighten the valve lash a few thousanths.
     
  21. trapster

    trapster Well-Known Member

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    I would check your rim temperatures after a long drive. Hot rims could indicate you have a bearing issue or rubbing brakes.
     
  22. KosmicKuda

    KosmicKuda FABO Gold Member FABO Gold Member

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  23. 66fs

    66fs Well-Known Member

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    Try this first.
     
  24. GenLee1970

    GenLee1970 Well-Known Member

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    I've been through the winter gas mileage thing with my slant a body, especially coming out of a 95 Saturn that would get 35 mpg year round. I googled slant six mileage figures and came up with the same figures you were expecting. And those figures ARE possible.

    The fact that you mention that a new thermostat helped keep you warmer while driving indicates it's cold where you live. Winter is the worse time for fuel economy, especially in these older cars. The colder it is, the worse it is. The technology in your car is 60 years old and very crude. That's why the car companies went with computer controls 35 years ago. The first few miles of driving in really cold temps use the most gas. The further you drive once it's warmed up, the better your fuel economy average will be. But if you're driving only a few miles to work each way, the car is subject to 2 dead cold starts a day to cover very little distance. Do this 5 days a week and add in a trip or two to the corner store to get beer or smokes and 12 mpg could be the painful truth. Once the temps warm up it'll be much better.

    I've gotten some really great figures with my car by getting it running great (with help from this forum) and driving very carefully. 24+ on the high end and high teens on the low end in warmer weather. Average summer driving was about 21 combined, but the average is much lower in the winter. It started driving me nuts so I stopped topping it off and tracking the mileage and just try to enjoy my car now that I know it's running it's best and what it's capable of under the right conditions.

    I have gotten better than 24+ on long trips but those figures seem to be an anomaly. Not all filling station pumps shut off that the same time. But high 23's and low 24's were a very repeatable figure.

    Driving style can't be emphasized enough, btw, as others have mentioned.

    Once you think you've got it all dialed in, drive it to a gas station (it'll be warmed up by then), top it off and then get on a highway or nice winding back road where a steady speed is possible. Drive carefully and easily for 100 miles and refill. That'll show you what the car is capable of under the best conditions.
     
  25. WRB426

    WRB426 Insane 4 Mopars FABO Gold Member

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    Drive a V8 and count the smiles per mile.
     
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