i have a '97 f-super duty w/ the 460/auto. the trucks weighs 9400lb as per the scale at the dump. i was/am not expecting anything great out of the truck, but i think i should be getting better than 4mpg. it takes me 1/2 the front tank to go less than 35 miles. i drive it very easy.

truck has been fully "tuned up"; plugs, wires, coil, cap/rotor, filters, etc. truck seems to run very well, only has 55kmi.

i am thinking an exhaust upgrade is in order: replace from cat back w/3" pipe and a flowmaster, and replace the puny dual 2" stock exhaust the truck has now.

am i wrong to think i should at least get 6+ mpg?

 

What kind of rear end gear do you have? 97 F super duty should be a mass airflow system, have you tried cleaning it? CRC makes an excellent MAF cleaner. FWIW, I was getting 8.5mpg with my 86 towing a 30ft 5th wheel. Total weight was 16120lbs and I have the air drag of a 12ft 3in high trailer. The 86 does have one of Ford's better exhaust setups, dual 2 1/2" pipes to the muffler, which is just forward of the axle, then a 3" tailpipe over the axle and out. You're down in the 454 Chevy area.

 

i *think* i have 4.88 gearing; its not something typically found in "pickup trucks".

i keep seeing reference to this mass air/air density/speed density stuff. can someone explain this to me, if it is even relevant to me?

my peterbilt w/a 3406 pulls 150klbs and gets over 3mpg doing it, so i think i can do a little better than 4

 

I think you should take it to a part store, borrow a code reader, and ask the computer what it thinks is happening. You have a computer-controlled engine, let the computer help you in figuring out what's happening.

IMHO blaming things *****-nilly (exhaust) isn't gonna get you anywhere since you have no hard data telling you this is, in fact, causing your problem.

Besides, I have a hard time seeing how the factory exhaust system - that works for everybody else - could cause your symptoms without there being other symptoms cropping up - e.g. lack of power above a certain RPM.

JMO.

 

Well, not to make this complicated but there's more factors to your MPG than you can shake a stick at. Biggest factor is condition, sounds like yours is in relatively good condition and well maintain, so that's probably not hindering you. Next is driving conditions. Are you doing city or hwy, and whats your cruising speed? If you're stop and go for 35 miles, it takes a lot of power to motivate 9400 pounds, finger of god 460 or not. Then gears, if you have 4.88 gears and you drive 70 on the free way, that's screaming in a 460. What about tire size? if you have huge over sized tires and you're a city driver, even more motivation it will take, but the stock size with 4.88 gears @ 70, again screaming. Give us a little more info and we'll see if we can't get you back in to double digits.

 

Nope.. if this is the old body style truck is should be speed density with an OBD-1 EFI system, trucks over 8500lb GVWR didn't get OBD-2 until the new SuperDuty was introduced in 1998.

But regardless of that with dual tanks there is always the possibility of the fuel transfer problem where fuel is returned to the inactive tank as you drive thus making fuel consumption look much worse than it actually is. To test for it run a tank dry(of just about) and then switch to the other and run for about half a tank. Then switch back to the first tank, it should still be empty of course, but if there is gas in there now you have the transfer problem and the only way to fix it is by replacing the fuel delivery module inside the tank. BTW.. this can happen in both directions so you'll have to test both tanks.

 

Was curious with the dual tank, one filling the other, lets say that while using the rear tank, it fills the front, If one were to fill both tanks full and only use the rear, wouldn't it only be able to fill the front so far, and then we could get an idea for actual mileage, without having to go back and forth, or do these tanks lack an overfill prevention that would allow it to overfill?

 

Nope.. it'll just overflow it out the filler neck and into the EVAP system as you drive.

 

yes, this is an old "square front" truck. my driving conditions are like this: no freeways, no stop and go. just a pretty consistant 35-55mph on rural roads, no traffic. i doubt ive ever driven it over 60mph. tires are 235-86-16, highway type tread. truck has a utility bed on the back, but is not taller than the cab.

i have also thought about the tank situation. ive never had dual tanks before. the gauge *seems* to work accurately for what is in the tank, based upon what the pump tells me the amount i just put in. i usually run the front tank down to just above E, and refill. i keep the rear tank as a "reserve" .

i knew that this thing wasnt going to get good milage, but i seem to be getting 1/2 what is normal. if i could get 6-8mpg, i would be satisfied

 

With 4.88's and that size tire, 55-60 is about all that truck should go, without really compromising mileage. Does it drive relatively well, doesn't feel like brakes are dragging, rear end isn't singing? Any check engine light? Do you have a tach in this truck?

 

If it's OBD-1, you can check codes yourself, here's how:

Fuel Injection Technical Library » How To Run a Self-Test
Ford Ranger/Bronco II EEC-IV Testing

The computer is aware of things going wrong, even if no Check Engine light is lit.

 

truck seems to drive fine, straight, rolls well. i can push it on a level floor. all tires wear fine. no smoke, no smells.

no check engine lights, no tach. truck passed NY inspection in august.

 

I didn't see anyone ask so I will. How are you calculating mileage? The gas gauge isn't a precision instrument and neither is the odometer. In fact as the tires wear down and go bald that alone can throw off calculations by ~1%. Pick a gas station and find a time when it's consistently slow. Go to the same pump and the same time every time you fuel up. Use the same setting for the little flip-down tab on the handle that holds the pump on and don't top off, just go until it automatically kicks off and then leave it. Keep very close track of the miles traveled and the gallons purchased for at least 3 fill ups to get a good average. After calculating you mileage that way we can see how bad the problem really is or if there even is one. It's good that you're only expecting 6-8mpg, it may be capable of 10 but I would guess that would be tops. Also check for fuel transfer.

 

Assuming you don't have a fuel transfer issue, and you do really have 4.88 gears, unless you've done better before, 4MPG may be all you're going to get. Now you'll get that empty or loaded down, but 4.88's would be overkill even for a large RV, 4.88's would be more appropriate with 19.5 wheels or a large off road tire, like 37+. I would say get a tach, it would allow us to see two things, what you're final RPM is locked out in overdrive, at speed, as well as we could verify you're trans is getting to overdrive and locking up in it. It would almost be a sure thing you have a 4 speed E4OD, the 3speed C6, although robust, I'm pretty sure was done away with by 97. Do you have a 4.88 rear axle code in the door jamb, or did someone just tell you that you had 4.88's?

 

Easy check as to which system you have is to look at the air inlet system. If it has one large hose from the air cleaner to a "Y" then it is mass air flow, if it has dual hoses, then it is the older speed density system. Speed density has a sensor on top of the heater or AC casing under the hood. If you look at the label on either the driver's door post or back of the driver's door (crew cab) it will have the axle and transmission codes.