I know these trucks get 8 to 12 mpg, but is there a reason or cause for this? My 79 F100 with the 400 , 2bbl, gets 8 to10 mpg (2 wheel drive, C-6, 2.75 gears).
I also have a 1987 motorhome (chevy, Sorry!) 454 4bbl that weighs in at 12,500 pounds and can pull a 2500 lbs car and still get 7 to 8 mpg.
What is the cause of such rotten mielage?
Looking forward to the resposes, Thanks.
Kevin

 

A C6 tranny significantly lowers gas milage... other then that, someone more experiences may be able to give you a better reason

 

No OD transmission, poor aerodynamics (shape and panel fitment), engine tuning are the three that jump right at me.

 

After 30+ years, it's possible that all the emissions equipment has been ripped out, leaving the engine an out-of-tune mess (just a guess).

 

I agree with that being one of the main causes.

Another big thing is they have the aerodynamics of bricks pretty much as SRT77 said.

Another cause is draggin brakes.


I have my 73 F100 still setup and tuned like Ford would have done it. Factory air cleaner with tubing still hooked up that runs to core support. All the vacuum lines are hooked up and the temp controlled vacuum switches. Egr is hooked up and working. Carb and choke are set properly. I wouldnt trade my little 302 for nothin. Gettin 20 plus mpg on highway.


I know your engine is bigger but you also have a c6 which takes more power to turn and if original your 400 has a retarded timing set.

 

OK, my truck has all emmisions, rebuilt carb (tuned) on Edelbrock allum intake, straight up new Edelbrock timing set, dual exhaust (with converters). Etc.
My 1987 12,500 lbs, motorhome has the aerodynamics of a 2 story building of bricks, 3 speed (th400) tranny, 454 4bbl all stock, all emissions, and towing 2500 lbs. It will consistantly get 7 to 8 mpg. This thing turns 2900 rpm at 60mph. Now that I think of it, it gets the same or just a little better without the car in tow.
So were talking a lighter smaller truck, less cid, 2.75 gears. This is why am curious , and why it just doesn't add up in my head. Not that it is fixable, if I could get this truck to get a consistant 18 mpg, I would be a rich man selling the process, Thanks for all the responses, its nice getting other's thoughts.
The dragging brakes sugestion has me thinking of at least checking them.

 

first there heavy as hell and gas has changed over the years plus the m series motors generally have a very low compression ratio that doesn't help. Plus your gears are probly so high that it takes a lot to get that big land yacht moving.

 

I think if you would use an advanced timing gear set and 4bbl with a new intake, you would see a good bump in your mileage plus a significant increase in power.

 

I've got a 79 f100 2wd with a c6 and a 302.3.00 gears. Edelbrock intake and holley 650 carb. I average 14mpg and i can get 18 if I start off easy and stay under 60.

 

well one you have mentioned twice is the 2.75 gears. Those would be fine if your running 70+ all the time otherwise your lugging the engine and making it work harder. the motor home probably has better gears that keep it in a more efficent band.

And two that want to make the claim the C6 is the issue, the TH400 draws just as much power so that's a not even an issue deal.

Also I had a 74 F250 with a 360 and a C6 got 11mpg had a 73 with a 360 and a 4spd got 11mpg. I know the automatic haters want to bash them all the time but they really don't make a majjor drop in mileage (if they did, then they never would have taken hold in the 70s during the oil shortages)

Also there is a reason you didn't see many motorhomes with 400 fords in them... They were underpowered and poor on economy. hate chevy or not the 454 is a pretty decent engine especially in applications like a motor home.