I got my truck running and driving again and am wondering what you 351/400M guys are getting for gas mileage? The truck is a 1978 SWB 4x4 with a Mile Marker conversion in the NP205, a 351M C6, and 35 " Ground Hawgs . I drove it to work Friday and went through 1/2 a tank of fuel in 50 miles(mostly country roads @ 55 mph), that's about 6 mpg. I have replaced the timing chain with a straight up double roller and it has a Holley 2300 2 barrel. I do have the vacuum advance hooked up. It has new plugs, wires, etc.

I can't believe that I will only get around 6 MPG from this combo . I don't remember my '79 which was basically the exact same truck except a 4 speed getting this bad of mileage.

 

you need to clock you miles and fill the tank back up to see how much you ACTUALLY used

when my needle is at E.... i still have 10 gallons left in the tank.... some are horribly inaccurate. i get 10-12 locally with almost the same setup.

 

Never mentioned the gear ratio but 6-8MPG is about average.

 

topher- I have the same truck as yours, owned it for over 25 years, and the best I've ever seen (when it was a youngster) was about 12-13.
It's back to stock suspension and height now on 31x10.5's, mildly built .030 over 351M, headers, Weiand and 1405 Edelbrock. I'm feel lucky if I get 8mpg; the gas gauge doesn't work correctly, so I note the odometer reading and put fuel in it after 100-120 miles of use. I've driven it once recently where it was on the highway only, and it got 11. It's not my daily driver.

 

I checked the mileage tonight and it is 8.07( 70 miles 8.674 gallons). The gas gauge is way off ,it indicated just over 1/4 tank. How much do you guys think the bias ply Ground Hawgs are hurting me? Would a swap to and Edelbrock 4 barrel help any due to the smaller primaries? Would an MSD box help? I need to get this a little better, like in the 10-12 range, so I can give my '85 Mustang GT a little TLC this winter or it will eat all the 'Stang funds up in gas!

BTW the truck has the stock gear ratio, I don't know what it is though.

 

Before I hit E i`m out of gas learned that twice..

 

These things are bricks first off all, and mpg's aren't one of their highlights. I've always gotten somewhere between 6 an 14 mpg's out of all my rigs with 351M/400's...depeding on gear ratio, trans, engine mods, etc. Typically, I got the best results when I just dealt with carb/intake and headers/exhaust...and left everything else alone. Once I cracked open the engine, the mpg's just seem to start dropping. That being said, I do think swapping to a 4bbl carb/intake will help you...in theory...you just have to keep your foot out of those secondaries.

 

You probably won't get much more than that.I'v had three different F-150 S.C.s ,one with 351m-2/bl. ,one with 400 -2/bl. and one with 460- 4/bl.
They all had 3:50 rear gears and same size tires.All with c/6 transmissions. All used a lot of gas, when they were allmost new,12 to 13 , later 9 to 10 . The 460 was the best on mpg even with the bigger carb. They were all used for the same job under the same conditions all being used at the same time . Different drivers,but all parked at the shop at night. Hauling construction tools and misl. building materials. So ,yes they like gas. I had a 79 bronco at the same time and it was about the same even after I changed it to a 460. JIM

 

i have a 1978 ford lwb f150 with a 351m. it has a holley 4412-2 2 barrel on it, 33s tires with no lift. i get around 7 miles a gallon. im going to switch to a 4 barrel here pretty soon.

 

that sounds pretty low...my speedo doesn't work, but I drove a few cities over...50-55 miles total and put in $20 at 3.25 a gallon give or take...when I got back home, I had a slight amount of fuel to spare...so by rough calculations that's about 9 -12 ish...75% highway 25% Street...3.50 gearing, 33" tires..dual exhaust, everything else factory stock but in a good state of tune...carb dialed in real lean on the idle screws.

 

my 79 f250 ranger has a 351m its stock has stock 2bl
c6 what i believe to be a np205 i have to look at the gear ratio to tell you
it has a 4'' lift and 35'' bfg mud terrains gets 7 and 11 is the highest thats with feathering the pedal


my 79 f100 custom stock 302 has an rv cam, stock 2bl c6 trans 9'' and i think 3.25 rear end gears in the 9''

it gets 10 around town highest around town is 14

freeway can be 10 one day and 18.8 freeway and a little town driving this weekend

highest i got was 23+ mpg on a 75 mile or so trip

 

What does S.C.s =

 

dude there going to drink gas. when these trucks were built they were built to build the U.S.A. and thats just what they did. gas mileage just was't a real concern at the time because folk were making good money but mostly the contracter of the time and just about all of them had at least one ford as there work truck. so drive it with pride.

 

Check your gear ratio.... I bet your fairly low, and getting all you will from it...

Then again, depending what it is, you might have a problem; cranking those 35s behind a C6(rpm/torque loss) with too high a ratio will mean your over reving to get moving and burning too much gas... at the same time, too low and you have too high an rpm at top speeds.

I had that problem with 33s and 3.08 rear end in my bronco... the high ration and low rpm at cruise wasn't enough to offset the excess gas i had to burn to get moving with no torque... Dropping the ratio to 4.10 so I could accelerate better gained me 1.5 mpg, even though it now cranks higher at highway speeds. In hindsight i should have gone with 3.75s and probably gained another 1 mpg. I get 7.5 as it is, stock 400,c6,205 and those 4.10s and 33s. Only down side is I miss being able to go 130mph and only crank like 3000 rpm.. ;-)

6 to 8 averaged city/hwy with 35s and a good ratio, is about right in my experiance.

After a full tune up, clean carb etc, I can get 12 from my 250 w/400, but thats stock tires, low ratio and no city driving.. add in city stop and go, and a bit of wear on things and I'm averaging around 9.

I've gotten 18 out of it before, but that was highway only, and with just enough running problems to keep me from flooring it off the line, but running great at high rpm... it kept me careful, and while running terrible, got great millage...

If you think 6 is bad, you don't want to know what my 76 150 with a 460 gets... I don't even want to know..(closest I've ever clocked it is 6.5mpg, generous, highway only) I hate to think what its going to be with a good tune up.(it'll either run better=better mileage, or run better and I'll hot rod it more and get like 3mpg!) Its got a 3.25 rear end, 2wd with a c6... I don't know if I want to try lowering that to gain mileage or not... it might hurt it more than help.

G.

 

S.C.= Super cab or since i had three, it's cabs. About a 30" space behind the front seat for cargo space and two jump seats on the sides.

C.C. = Crew Cab full size rear seat and four doors.

Some say " extended cab" but that could be crew cab or super cab.

I'v never heard them called any thing different.