Ok I swapped out my old 400 for a freshly rebuilt 351M. With the 400 I was getting 11 mpgs...with my 351 Im getting 8.

Stats on both engines:

400:
cracked rings
137,000 miles
4 bbl edelbrock carb and intake
stock besides that
dual exhaust

351:
fresh rebuild
mild RV cam
same 4bbl intake and carb
dual exhaust
bored .040" over

Both engines are tied to my C6 and my 4:11 rear axle. This is in my 78 F250.

Question is this: How can I get that mileage up? Vague question I know but I think that 8.4 miles per gallon is kinda low...

 

Attempt any carb tuning? That's where I would start.

Other mileage gainers:
Driving style
Tire pressure
Tire sizing
Weight reduction
Rear gear change
Aero improvements
EFI (Atomic, FAST 2.0, E-Street, Terminator, etc.)

 

Yup I tuned it to run a bit leaner. That mileage is also after I've granny driven it off the line. Speed also has been under 50 mph since ive only driven it to work 10 minutes away

 

How many miles have been logged on the rebuild??

 

Guy before me (one that rebuilt it) says 3-5K on it. Ive personally put about 140 miles on it

 

The problem could be in the RV cam.

 

400's always got better mileage that a 351M. The low end torque gets them around without having to turn up the engine. That's just the way it is. Realistically, you should be getting better mileage. Your gears are killing you regardless. Steep gears equals rpm's on the highway. Rpm's means the engine has to spin faster to maintain speed. Lower the gears, (numerically lower, higher ratio) lower the rpm's lower the fuel use.

.....and let the engine break in a little. Also do NOT use gas with ethanol.

 

With my new rebuilt 400 on a c6 and 411s stock carb and manifold f250 4wd I was getting 6mpg. After I got a few thousand miles on it it is up to about 11 mpg.

 

Well thats good to know. Hopefully once its broken in it'll do better

 

Your truck is punishing you for swapping out the mighty 400 for a cursed 351m

 

Quite possibly....my truck ha hated me since the day I got it...

 

Then again I wanted to toss a 460 in there...almost makes me wish I would have

 

I don't think it's gonna markedly improve with that kinda mileage already on the rebuild. It is pretty much safe to say that it is beyond broken in.

 

Thats what I figured... Would ethanol free gas help?

 

Well look at this this way, at least you are getting the 460's MPGs.

With 3-5K miles, the rings should be seated already. Did you check engine vacuum for leaks or possibly other tell-tale signs?