I have a 1976 f150 4x4 with a 360 NP435 4spd with 3.55 gears. I get about 8 to 9 MPG just using it on the farm but I really want to make it a DD so a 300 sounded like a good motor.
I understand that these trucks have all the aerodynamic engineering of a brick but I wanted to know what kind of MPG guys get with 300's in 4x4s. The 300 that would be swapped in is out of a 1966 M250 Tremec 4sp with no bellhousing and is probably just getting a deck+rings+hone to clean up the bores and true up the block. It doesn't have much of a ridge on it and made good compression when it was pulled a decade ago so I figure its fine but are there any cheap ways to gain power/economy? Are the heads the weak link or is the 1v carb the biggest restriction? I plan to port the heads for a little more flow but the more I look at it the less I want to port something that restricted.
Ultimately I'm just looking for more MPG, parts availability and want to match the 360's power is this unreasonable to expect from a mild 300 with a stock cam?

 

F-250 Restorer would be your main guy to ask, since he swapped the 360 out of his truck for a built up 300!

But with your gearing and transmission, you should be able to get an honest 14 - 16mpg (maybe more) with a 300. When mine was stock, I used to be able to get 22 with my 1bbl 4x4 300.

The head, intake, and exhaust are all very restrictive and addressing all three will wake the 300 up very quickly. There's lots of posts to the point of redundancy about performance mods for the 300 here. Plenty of reading with a quick search. Four barrel upgrades, exhaust upgrades, head porting, etc.

Have fun, and of course, take pictures.

 

Unless you mod the 300, there is no way of coming close to the power of the 360. My 390 was rated at 250 hp with a 2v carb. An oem 300 is about 120 hp. But you can easily reach the hp of the 360 by porting the head, installing a good cam and kit, intake, 4v carb, high flow exhaust, a high power ignition.

The good news is that having done those mods, you'll still be getting about 15 mpg. And, by checking around and being smart, you can buy the parts and do the mods w/o spending a fortune. My goal was to retain the hp yet get 50% better mpg. And I did.

Of course going from an FE to a 300 also means a trans change. Just changing to ZF trans, or another with OD will of course help your mileage.

 

Oops, you had a 390, not a 360. =X

 

I thought it was the other way around...50% more power, with the same or better mpg.

 

Just as a 300 is a stroked 240, the 390 is a stroked 360. The block and them thar things are all the same.