Hi there,

I have a 77 F250 Highboy with a mild 400 in it and I am consistently getting 2 mpg. I have adjusted the carb for my altitude, replaced all the plugs and wires, replaced the PCV valve, adjusted the idle and it doesn't help. I know I should be getting 10-12 mpg so I am completely baffled about what is wrong. I would really appreciate any help you might be able to offer. I love this truck but the gas mileage is absolutely killing me and I cannot figure it out at all.

Here is some info on my build in case it is useful to you all.

400 with 55,000 original miles on it
Edelbrock 600 cfm carb
Edelbrock intake
Hedman Headers and Flowmaster 40 mufflers
4-inch suspension, 3-inch body lift
39s at 25 psi
parking brake is off (haha)
TCI C-6 transmission
MSD ignition stuff all the way through
K&N air filter

I don't drive faster than 55mph on the highway and they only have 93 octane out here.

Thanks a lot guys, really appreciate the help!

 

Is the vacuum advance hooked up ?

 

I dont know much about 39's, but that tire pressure seems low and causing more rolling resistance.......but not to the tune of 2mpgs worth

 

At 2 MPG I would expect it to either be belching black smoke out the exhaust or leaking fuel from underneath somewhere.

What do your spark plugs look like?

39" tires makes me question the accuracy of the speedometer and that PSI does seem rather low as already stated.

 

I agree with the mod on this one, sure sounds like a leak. My 79 on 44s gets around 7-8...
What gear ratio are you running in the rear? If the rpms are above 2500-2700 ( give or take) at highway speeds those motors will suck the gas out like a water hose.

 

Yes, it is. That is supposed to help with gas mileage, right? I am a newbie BTW, doing my best to learn all I can about these trucks.

 

It was definitely running rich when I first arrived out here, because it went from nearly sea level to just under Denver height. I changed the needles and jets on my carb and from my layman's perspective, it is no longer running rich (no smoke, doesn't run like a bike with the choke on..haha) I will check a plug tonight and see what it looks like.

I noticed that you are in the Denver area, in case you are running an Edelbrock carb, could I trouble you for the needle and jet numbers you are using? I have done all sorts of searches on this site and online for this but came up dry.

Thanks a lot. I live in Tanzania and am completely on my own out here with this. A labor of love, I tell you.

 

Okay, definitely going to check for a leak tonight! Where should I be looking for the leak? Around the carb and the vacuum advance unit or on the intake manifold?

Going to add a bit of pressure to the tires and keep the rpms down (wish I had a tach). That might be what is killing me. My speedo says 40-45 but that probably means more like 60 mph and my guess is that my rpms are well above 2500.

I have stock gearing in my Dana 60 in the rear.

 

A leak is a definite possibility. You probably should check your entire fuel system, all the way from the fuel tank clear up to the carburetor connection. Start with any rubber fuel line segments you may have for deterioration or loose clamps, these might be your first suspect. Check your fuel pump, make sure it's not leaking for whatever reason. If no problems are found in these areas, you may then need to closely scrutinize any and all steel lines. Hairline cracks or microscopic pinholes may only allow leakage while you are driving down the road - making these type of leaks somewhat hard to find.

Good luck, hopefully you find a cure, 'cause 2 mpg definitely sucks!

Paul .

 

I definitely agree with checking the entire fuel system front to rear; Colorado is an exceedingly dry climate and rubber dry rots, I ended up replacing all my rubber fuel line components because of that. There are pieces of rubber fuel line between the tanks & steel lines, mine on the rear tank (on my 1981) was so bad it was literally crumbling to pieces.

And there are rubber sections at the tank switching valve (if equipped).

Also check your engine oil, the diaphragms have been known to leak gas into the crankcase.



I have a Holley Truck Avenger, the general rule of thumb is to reduce by one jet size for every 2,000' in elevation above sea level.

I don't remember on which size I settled but I *think* I ended up only one size reduced from the out-of-the-box configuration, I seem to remember settling there because the spark plugs showed it was running lean.

I haven't connected to an exhaust sniffer to verify anything, am going mostly on engine behavior and spark plug condition.



In all cases, though, jet size wouldn't be the cause of 2 MPG, something more serious is wrong.

One can certainly *smell* in the exhaust when it's running rich, and a carb tuned for sea level operating in a high altitude will certainly smell that way. But it won't be making black smoke; if it were the case that black smoke results from gaining elevation, half the cars here in the 1970s (before EFI) would have been belching cuz they were out-of-state visitors.

Well, not half, that's an exaggeration but I think you understand.



My truck with a mild 400 runs around 3,000 RPM at ~65 MPH on the highway and gets 9-11 MPG.



You might start your engine and look down the throat of your carb, see if you can see fuel spilling or dribbling inside, perhaps a leaking float valve or sumthin.



I suggest keeping a fire extinguisher in the truck, what begins as a small engine fire will quickly eat up the whole thing.

Windsor truck fire - YouTube



There's more video available of that event, see:
What if your car caught fire? - Four Eyed Forums

 

Ctubutis,

Thanks a lot for your help, man. Will definitely do what you and others recommended and that is look for a fuel leak. You guys are so right. No effing way that 2mpg is not a result of a leak. Didn't realize that some leaks only take effect while driving so I will have to inspect with earplugs and while someone is on the throttle somewhat to see. I really hope I find a leak because that will be a relatively easy fix!!

I went down a needle on my Edelbrock too, but like you said, that really isn't going to be the reason my mpg is one fifth of what it should be.

Thanks to everyone!

 

This is probably a combination of factors. The tire pressure looks way too low for street use. What is max recommended? Fill them up. Second on the list what is your final drive ratio? With 39's you are going to need a low gear to get the thing moving. The higher the gear the more effort to get it going. This plays more of a role if the majority of your driving is in town. I would definitely be in the 4 range on gears. As stated leaks. Dents are famous for tank leaks. Do you have single or dual tanks? The rear tank is good for forming holes above the straps and for some reason mid tank on the front side. Front tanks as well. The rear tank problem plagued Ford trucks all the way into the '90's.

Simply looking under the truck at the tanks should tell the story there. They will show a stained pattern from leakage. Keep in mind a tank leak can occur anywhere meaning if in the upper portion of tha tank it will only leak until below the hole. Has anyone been into the tanks? A rolled sending unit seal will leak. An open into the tank can also cause evaporation. If the seal rolled it could be causing a leak when filled along with evaporation all the time.

Vac leak but I would assume a large one. I have seen vehicles run with pretty big ones that didn't seem as though it was a vac leak. You have verified you are not in 4 wheel drive I assume. Verify. Just because the shifter says 2WD doesn't mean it is. Just because your hubs say free doesn't mean it is.

 

39's with 3.54's?! There goes your gas mileage! Lol.

 

Ok I have been through this multiple times. I haven't found any reference to a ratio. Is that the factory highboy ratio? Something I thought about. Check the brakes for drag. I would be thinking the fronts more than the backs. Fords were pretty famous for pistons not returning in the calipers along with if not greased not sliding on the mounts when they should.

 

I can't even imagine somthing using that much fuel that five times the fuel burn you should expect. At 2mpg with the stock 20g in-cab tank you would have a 40 mile range