Slant 6 Gas Mileage

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Mexican Spec

Mexican Spec

Mason66
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I have been checking my milage. I have a 1975 Dart slant 6 three on the tree. I have been getting around 9MPG.

What are you guys getting with your cars?

I feel like I should be driving a 1969 Ford LTD with a 429 V8 with these kinds of numbers.
 
Oh boy, here comes the. Few with "I get 33 on a bad day",
"I get 55 when she is tuned up good!"

Go through the basics and be sure to check things like fuel pressure, carb float setting, and timing!
 
rumblefish360 said:
Oh boy, here comes the. Few with "I get 33 on a bad day",
"I get 55 when she is tuned up good!"

Go through the basics and be sure to check things like fuel pressure, carb float setting, and timing!
Click to expand...

Engine is new, well rebuilt, and the carb too.

The mechanic can't figure it out.

I get better milage with my '74 Dart 318 2 BBL, and Imean much better.
 
I get 16-18 around town, 22-24 hwy with stock intake and 1bbl. I would double check carb, timing, and valve adjustment.

Are you running points?
 
Well not sure what to add. But are you sure you are figuring it correct? Are your brakes dragging? What kind of numbers did you get before the rebuild? Drove mine 350 to Rod Run in Pigon Froge TN last year and got 23mpg. stock bottom end, Holley 4bbl with a header. 3.73 gears in an 8 1/4 rear, with the A/C used part time.
 
rumblefish360 said:
Oh boy, here comes the. Few with "I get 33 on a bad day",
"I get 55 when she is tuned up good!"

Go through the basics and be sure to check things like fuel pressure, carb float setting, and timing!
Click to expand...

I know and they are all so fullashit. Either they just like lying through their teeth or they don't know how to do simple math. I never seen one get better than about 25 in anything.
 
You have something dragging you down thats for sure!! Brakes, bad alignment, rear end..
I get 20 to 24 miles to the gallon easy, what ignition and plugs are you running ?
I hope the new build loosens up some, but dang!! 9 mpg park that puppy on a level
parking lot and but it in neutral and push it and see what it does and sounds like..
Carb has got to be running fat/rich to eat that much fuel or it is running under a load of some kind.
 
Like the others, I get 23ish (highway) with the wife's 73 Dart. Nothing special going on. The only mods are the removal of the timing delay crap, tune up parts recommended on slantsix.org, and the fuel line mod.

If you are getting 9 mpg, something should be obviously wrong.
 
70 Duster Slant 6, brand new carb, before I put electronic ignition on it, we went from Lynnwood Wa, to Long Beach Wa, about 180 miles, at 65-70 mph on the freeway, used 1/4 tank of gas.
 
I just read that the engine is newly rebuilt. Double check the cams timing events to be sure it is installed as per the manufacturer. Yea, I know that is a lot of work.
 
I had a remanded bbs that ran really rich. Nothing I could do would lean it out enough. I tryied 2 1920s but they leaked. I found a junkyard 1945 and capped off the emission ports then jetted it up a bit. Runs good. The hei conversion is well worth it also.
 
rumblefish360 said:
I just read that the engine is newly rebuilt. Double check the cams timing events to be sure it is installed as per the manufacturer. Yea, I know that is a lot of work.
Click to expand...

I was thinking the same thing. Try putting a vacuum gauge on it and see what reading you get. That could tell a lot.
 
What are you using for your miles measuring? Try it again with a GPS.

then what others have suggested looking at.
 
Must be towing a 10k trailer or trying to haul a tree. My / would get low 20's if I beat the crap out of it and still pull 30 on the highway.
Have to agree your cam is off a tooth, Timing is way off, Driving in 2nd gear, bad vacuum leak some where, carb is pouring fuel. bad coil not fireing the fuel, Distributor cap cross arcing inside. rotor with a hole burnt in it.
 
Could be a combination of a new, tight engine, and the timing being (way) low on it.

Points out of adjustment.

Rockers out of adjustment

Pull a couple plugs and read the tips. They will quickly tell you if its a tuning problem. A vacuum gauge is not a bad idea as well.

Does the engine have good power relative to before? You might also be looking at a cam gear being a tooth off.

Really, anything the good folks here have mentioned, or a combination of a few could be the culprit.
 
RustyRatRod said:
I know and they are all so fullashit. Either they just like lying through their teeth or they don't know how to do simple math. I never seen one get better than about 25 in anything.
Click to expand...
Feather duster was rated at 34 I believe.
 
Fst73_360 said:
Feather duster was rated at 34 I believe.
Click to expand...

I believe they also were rating the original Omnis at 48 or 50 mpg back then. It wasn't realistic.

Given the weight, aerodynamics, and gearing (and taking into account engine power), I'd say a realistic max is 28 mpg.

I'd expect a slant six to get 15/22 with an auto, 14/24 with a 3-speed and 15/28 with an OD 4-speed and the tall gears.

I have a 2004 Chevrolet Colorado 3.5L/5-speed stick 2WD and 3.42 gears and if you're really nice to that it will get 27-28 highway. Its probably more aerodynamic than an A-body, but is a bit heavier.
 
..with a rebuildt carb and tuneup, we could never do better than 17 (Cdn) mpg. And yes, the canadian gallon is larger than the american one..4.6 Ltrs vs 3.8 ltrs..

Ian.
 
The car you have ought to be getting 22-24 mpg on the road and a bit less in town. What rear gear ratio do you you have? That might make it a bit lower but not much.

No engine has that kind of internal drag even when first started to drop that kinda mileage. It hasa to be carb or really bad timing or somethign of that nature...I am betting the carb is way out. The choke sticking on would do it.

Real mileage on the '76 Dart Lite that I drove over 100k miles was 28 mpg highway at 70 mph; it would get 29 mpg in the midwest. It had the 4 spd OD trannie and around 3.0 rear gear. All stock.
 
Just out of curiosity, what kind of numbers do you get in a compression check? Those ought to point to either internal problems with the engine or external "tuning" type problems.
 
Explain exactly how you determine mileage, for both the slant and the 318. If you are using the marks on your fuel gage on the dash in any way, that is too naive to even discuss. Ditto for the odometer unless you verified it against a GPS or mileage markers on a long trip.
 
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