Just verified with 20 turn method...20 turns, 27 1/4 turns of drive shaft... 2.75 geared rearend. Feels lower...I'm turning 31" tires too. Guess the 400 is torquey! lol.

So what is amiss here. Maybe my timing is way off...or need a good tune up or different jets in the carb. My tail pipe is very sooty...I was told with todays gas that can not be a sign of anything...I think he is wrong. SURELY with this rearend and these tires I can do better than 9mpg.

 

probably not. Have you upgraded the cam? These things are set retarded from the factory and if you have never upgraded the cam there probably isn't much else going on.

Headers would help with the breathing but will they get you over 9 mpg? Not sure.

 

Now that ya know what ya got to work with, the next step is to look at the options. As always, there's a cost/benefit relationship with what ever may be attempted. Some quick ideas:

• Go on a diet... Remove whatever is not needed. A loaded tool box does not help. Easy mods is to remove the tailgate and rear bumper. Out of curiousity, I just weighed my 70's dock bumper and it's 80 lbs!
• Be mindful of your driving style - anticipate/coast to red lights and stop signs, accelerate smoothly, and avoid extended idling.
• Auto trannys are inherently inefficient with slippage and heat. A manual tranny (1:1 fourth gear) may eke out maybe 1 to 2 mpgs. Overdrive? A 2.75 is too low (numerically) for OD unless all you have around is flatlands. Even then, the engine won't be working at its most efficient RPM.
• Aero improvements? These things are bricks and the frontal area is humongous. A possible mod is to block off portions of the grill that don't aid cooling. The radiator opening is smaller relative to the entire grill area so there may be a teensy bit of benefit. A 2WD may benefit from an air dam to channel air or block air from going under the rig and creating turbulence.
• Fuel injection - A definite possibility. Then the engine is getting only what it demands. Again, a cost ($) to benefit (MPGs) analysis is required. Run some numbers on how many miles would be needed before it actually starts paying back.... For example, at $4 per gallon and 9 mpg, it costs 44.4 cents per mile (OUCH). Assuming it gains 2 MPG with EFI, then a $2,000 EFI system would require about 4,500 miles to recoup the base cost... But 4500 miles at $4 per gallon and 11 MPG is still over $1,600 in fuel alone.

 

hey man ok if ur gonna keep the 351m ur gonna have to spend alot of money to get even close to what ur lookn for try a set of 350 gears out of a 4wd headers dual exhaust alum intake 650 eldebrock carb a decent rv cam n u might get 12 to 13 but for about 2gs is it gonna b worth it

 

Well I had posted some other threads about this but didn't wanna change the subject in them. I have a 400...with a 351m that is avaiable for parts. It WAS a hot engine...has dual plane intake...cam and such. This began as me wondering if it would be worth the time and money to put a 300 and small block C6 in....may be leaning towards that still. The 2.75 rearend would cruise right in the 300's torque range.

 

I have an 80 F150 that came factory with a 300, C6, and 2.75 rearend. I also have 31's on the back. It's a decent combo for MPG, but it's not the greatest for towing. It towed much better with the 235/75-15 rear tires.
MPG can vary between 12 and 16, depending on how I drive, what I haul, and how windy it is.
I will either go to a lower rear gear, or smaller rear tires.

 

i have a 77 f100 factory 300 3spd on the colum w a 300 gear ratio w 275 60 15 on the rear she gets around 17-18 mpg

 

As for towing power, well, I just home with this about an hour or 2 ago. Pulled with a 300, C6, 2.75 rear gear and 31x10.50-15 BFG's.

 

9mpg doesnt sound too far off for your 400 and 2.75 gears and 31in tires. I'm runnin a 351m, 2.75 gears, and 265/75-15 tires, and get about 13-14mpg, if i'm not heavy on the gas. If yer blowing visible black smoke, then i'd be concerned with tuning the carb to lean it out a little. It wouldn't hurt to use good quality plugs as well. I don't use the plain copper, but the mid-range/cost plugs. Something else i didn't see mentioned is that i really do get more mpg with real gas, not the ethanol mixed crap. I run pure gas in my vehicles only. It costs a few cents more, but my mpg is better, so the cost evens out. And the real gas does not eat away at my rubber fuel components, unlike the ethanol mix.

 

I get 12-13mpg with a 78 F250 SuperCab 400/C6 and 35's. Don't know what gearing though.