I have a 390 in my 79 bronco.
I get 8 miles per gallon maybe 9 on a good day but no better.
I have been told that it should get 12-13.
Sounds right on the money to me, and in my experience...
that's what you should be getting, easy.
I get >12mpg on my '75 F150
360FE+T18 -original- 2100 carburetor
3.50 gear, 31.5" 235/85-16E's
Comp 252/252 straight up
Stock manifolds, opened-up stock Y-pipe
Straight through 2+1/2" muffler and pipe
...and about 95k on the rebuild too!
Over 13mpg when it was first rebuilt. :)
My son is getting 12 to 13 mpg on a rebuilt 360FE
Unknown cam and miles!
We call it "The Farm Truck" it has a flat bed and really was a farm truck.
It was given to me by a cowboy buddy for scrap and parts. :)
Stock manifolds with dual exhaust.
It pulls stronger than mine is all we know about the engine.
'75 F250 360FE+T18 stock 2100 carburetor
3.73 gear 235/85-16E's
His -was- getting 8 to 9mpg before we rebuilt the 2100 carburetor
using -used- parts I had on hand. :)
It was a blown power valve messing up his mileage I believe..
The trick to good gas mileage is in the carburetor adjustment IME.
You know how to do that? :)
timing set correct,
vacuum advance is hook up and working right.
No brakes dragging.
33 inch tires with 3.50 gears
my speedometer is correct.
it does best on 87 octane.
Since there is for sure something wrong with it, then
-at least- one of those things in that list is a wrong.
Start over on your list of things "that are right"...
1) What's the actual timing?
a) At idle?
b) Full mechanical at what rpm?
c) Mechanical plus vacuum advance at what rpm?
2) Carter AFB? ;)
a) Who adjusted it?
3) "RV cam" covers a lot of ground.
a) Which cam is it -exactly-?
b) Is it timed straight up or retarded?
4) Cool that your speedometer is right. ;)
a) What about the stinkin' odometer tho... Is it right? ;)
5) It's cool to hear it's running good on 87 octane. :)
...and for-sure, it ain't pinging -any-? ;)
Alvin in AZ