So why does my old, giant wagon, with a big motor get bad mileage?

Hi Eric
Consider the vacuümsystem of the car,is everything close/tight included the brakebooster?
Do the valves in the cilinderhead close secure?
You can use/add as a cleaner Marvel Mystery Oil in your engine to clean.
Very often inside the engine there's carbon due age .
Use good oil,for instance Comma XT2000 ,no full synt !!
Ofcourse these cars aren't the most economic ones you know

 

It's been vacuum tested, and it seems solid. Since I have EFI on it, and so I can track the vacuum in realtime.
Thanks for the other ideas.

 

My dd will get between 38 and 42 depending on my driving habits. My 78 Ford with a 400 gets around 12. The best i got was 15 on a run. Other things to check, tire pressure, usually can add 4 or 5 pounds more to each tire.Check brakes make sure they are fully releasing and not adjusted to tight. Check wheel bearings. Try advancing the timing a few degrees. Check for exhaust leaks. Is the odometer accurate?A friend of mine has a 78 cougar with a mildly built 460 ave 16 all the time

 

The Cougar is a lighter car and the 460 has to work less harder in there. I had a Skoda with the largest engine option which was rated the most economical for that car. It had a 1600 cc engine and the other two availabilities were 1300 sized

 

FWIW, I have a Gen VI BBC in a 1991 Olds Custom Cruiser (4400 lb.), and it gets 6 MPG around town, and about 10 on the highway. It is actually a 461 with a mild roller cam, aluminum heads, Scorpion roller rockers, Holley MPFI, shorty headers, and Magnaflows. The gears are 3.23, and I have a 4L60 with a 2500 rpm stall converter behind it. The compression ratio is just short of 9:1. I run 93 octane because I do not have a knock sensor.

I can drive about 140 highway miles on almost exactly 14 gallons (Clearwater, FL to Daytona Beach).

Depending on the specifics of your engine, it may be the final gear ratio. A lockup converter would make better gas mileage, and an overdrive trans would also help, especially on the highway.

 

Thanks for the info Fred, good comparison points and ideas. I met a guy at work that has a late 60's mustang with the same engine, and he only gets 8-10 with his.
Funny thing: I've been driving it 'really nice': No hard acceleration etc, and have been getting 6mpg. For the next tank I did the opposite: Fast accelerations etc. Got 7mpg. Go figure.

 

Your engine probably uses more gas at idle than a Toyota Corolla at 60 MPH.

 

I bought a '76 LTD with a 351M to use as a running parts car for when I restore my '73 Galaxie police interceptor.
I was getting around 8 mpg normal driving, then I replaced the U-joints, and rebuild the front end (new ball joints, bushings, etc) and she went from 8 to 11 mpg... sometimes it isn't directly engine related, sometimes its the car struggling to get down the road because of worn out suspension parts.

 

That makes sense. Most people seem to not be taking friction into account. Not regularly controlling tire pressure is the most common mistake

 

My '75 Country Squire with the 460 will manage 14 mpg hwy. Something I did, which made a huge difference in performance was to ignore the factory timing marks and time it with a vacuum gauge. This eliminated a bad hesitation and gave it strong torque at all speeds. It is very reasonable on fuel for a 460 in a 5,000 lb, brick-shaped land yacht (about 12 mpg around town). The timing is advanced until the vacuum gauge reaches its maximum reading. At this point, the idle begins to get a little rough, then I back it off a couple of inches, or until smooth. Drive the car and see how she feels, then adjust again as necessary. It should not have any excessive detonation under load. Very slight pinging when climbing a grade is acceptable. I have also gone through all of the vacuum components and eliminated any leaks found. You should be able to pull 20 inches or so on the gauge at idle. If the vacuum is much lower, then there may be other issues such as hose leaks, intake manifold gasket leaks, brake booster failure, engine valve wear, etc. The '75 being a smog-tuned engine, I have also replaced the timing set with a zero-degree crank sprocket from a pre-1972 application and DuraSpark II ignition from a '78 LTD.

 

The thing about the "self learning" EFI systems is they need to be driven a lot to get dialed in. Driving 4 miles each way won't get it. Take it on a 3 hour trip with lots of variable driving conditions. You may have to do this multiple times. Freeway, back roads, up hill, down hill, sustained speed varying speed, hard throttle etc. I have a Holley system on my Chevy 400 that sets up via laptop. My tuner and I have logged MANY miles with him tweeking as I drove. Does your system control timing as well as fuel? That can make a big difference.
Pat

 

@oldsluvr & @nova66mussi : Thanks for those comments : to date I've not taken it on any long drives at all, probably should.
I believe my EFI controls both timing and fuel. I can only do so much: I can't really work on it myself due to a lack of garage, so I'm a the mercy of my local mechanic. Right now, since I drive it so little the mileage isn't a big concern, but I'm always on the lookout to make it better, and appreciate all the thoughts.