Miles per gallon

[delco1946]

I didnt see an existing thread about this but i'm curious what folks get for mpg for their setups. This might be helpful to know if you're in the right ball park when tuning your engine.

For example, my 1981 F250 trailer special with a 351m, C6 (3.55 geared) was built for brawn and beauty, not speed. I just passed emissions, so I know its fumes are in the "acceptable" category, and that this is likely as lean as she's ever going to get. And yet, I think i'm still getting like 9 mpg (maybe 10 if i'm lucky) around town and may that could go towards 11/12 on the highway (but that's a complete guess/hope).

Does that seem right? Or have I some "tweakin" to do? (Carb is rebuild, egr is new, spark plugs, wires are new, cat is new, etc.)

I did see a thread that suggested 460s are closer to the 6-8 range, but id love to know what y'all get in terms or mpg based on your engine and setup.

[1986F150Six]

1986 F150 with 4.9L and 4 speed manual OD = 15-25 mpg.

https://www.ford-trucks.com/forums/1325963-gas-mileage-recipe-4-9l-300-a.html

[Gary Lewis]

Dad said his truck, a 4wd F150 w/a 351M/C6 and 3.50 gears always got 10, or 10, and sometimes 10. My brother said his mid-80's F250 w/a 460/C6 combo and probably 3.55 gears got exactly the same MPG, even when towing.

My experiences have been different. Dad's truck with Rusty's "built" 351M and ZF5 got 14 on the trip from OK to PA and back, carrying furniture on the way and always on speed control - which hurts MPG. But I'll agree with Dad that the same truck with the stock 351M/C6 got 10 and was a dog in comparison to the other engine. Apparently Rusty's engine had a lot more compression and a decent cam.

And Big Blue, which is currently inhaling through a 4bbl, gets 11 on the highway running 65 - 75 MPH on speed control. But I think I could squeeze 12 out of it if I took it gently and kept the speed to 65 or less.

So if you are getting 8 or 9 around town and might get 11 or 12 on the highway I think you are probably tuned correctly. The C6 is power hungry and the 351M in stock form is badly neutered with a retarded cam and low compression, so you aren't going to get much better than what you are getting.

[ratdude747]

1986 F150 with 4.9L and 4 speed manual OD = 15-25 mpg.

https://www.ford-trucks.com/forums/1325963-gas-mileage-recipe-4-9l-300-a.html

Spring/Summer/Fall, once everything was fixed, My 1984 300 (Mostly Stock EEC-IV Feedback Carb) with an AOD and 3:08 gears (RWD, Stock tire size) was getting about 12MPG local driving and up to 17-18MPG on the freeway; it seems to be happiest 65-75mph, as below tends to result in a lot more downshifting (from what I've observed, faster does seem to save fuel???). Drive thru's and cold weather kill the fuel economy... hence why I took it off daily driver duty last night.

[grumpin]

With my 460 and C6 I’ve gotten 10-11 on the freeway, which surprised me, I expected al little worse.

8ish around town. Short trips and stop and go hurt, and 4WD for extended periods in the winter.

I know the original owner towed RV’s. I suspect there might be an aftermarket torque converter in the tranny and a shift kit. And the way it idles maybe an RV cam. These are gut feelings and I could be wrong.

[delco1946]

With my 460 and C6 I’ve gotten 10-11 on the freeway, which surprised me, I expected al little worse.

8ish around town. Short trips and stop and go hurt, and 4WD for extended periods in the winter.

I know the original owner towed RV’s. I suspect there might be an aftermarket torque converter in the tranny and a shift kit. And the way it idles maybe an RV cam. These are gut feelings and I could be wrong.

Oh that's a good point Dane, and I completely forgot that I was told upon purchase that a shift kit had been added to help with an automatic pulling trailers. Not sure if that helps or hurts gas mileage.

Gary - why do you say that speed control hurts mileage? I've only heard that cruise control improves mileage as its the acceleration/deceleration that is most wasteful. I know it tends to cause leaner firing and can heat the engine more so, but those also seem better for gaining mileage...?

[Gary Lewis]

Oh that's a good point Dane, and I completely forgot that I was told upon purchase that a shift kit had been added to help with an automatic pulling trailers. Not sure if that helps or hurts gas mileage.

Gary - why do you say that speed control hurts mileage? I've only heard that cruise control improves mileage as its the acceleration/deceleration that is most wasteful. I know it tends to cause leaner firing and can heat the engine more so, but those also seem better for gaining mileage...?

Yes, acceleration and deceleration is bad for MPG. I can see hills but the speed control can't, so it can't ease into the throttle as you start up a hill. Instead, it realizes the vehicle is losing speed too late and opens the throttle too much to catch up, and that uses fuel unnecessarily.

Similarly I back off the throttle as we start down a hill, as does the speed control. But I can see when the hill ends and ease back into the throttle at just the right time so there's no loss of speed. However, the speed control doesn't see the bottom of the hill and the speed drops off, so then it has to accelerate to get it back up.

I believe that the higher the horsepower of the engine the worse the hit from speed control. That's because the vehicle accelerates faster when the speed control asks for it.

As for proof in the pudding, Blue is an excellent example as he has turbos and a boost gauge. If I put him on speed control I can see the boost come on routinely when we hit a hill. And he'll get ~17 MPG. But if I feed him manually I can get over 20 MPG. I did so on a trip to Colorado a few years ago and after many hundred miles we were pushing 21 MPG - calculated manually instead of relying on the Lie-O-Meter on the dash.

[mat in tn]

Yes, acceleration and deceleration is bad for MPG. I can see hills but the speed control can't, so it can't ease into the throttle as you start up a hill. Instead, it realizes the vehicle is losing speed too late and opens the throttle too much to catch up, and that uses fuel unnecessarily.

Similarly I back off the throttle as we start down a hill, as does the speed control. But I can see when the hill ends and ease back into the throttle at just the right time so there's no loss of speed. However, the speed control doesn't see the bottom of the hill and the speed drops off, so then it has to accelerate to get it back up.

I believe that the higher the horsepower of the engine the worse the hit from speed control. That's because the vehicle accelerates faster when the speed control asks for it.

As for proof in the pudding, Blue is an excellent example as he has turbos and a boost gauge. If I put him on speed control I can see the boost come on routinely when we hit a hill. And he'll get ~17 MPG. But if I feed him manually I can get over 20 MPG. I did so on a trip to Colorado a few years ago and after many hundred miles we were pushing 21 MPG - calculated manually instead of relying on the Lie-O-Meter on the dash.

It is true that bullnose trucks are not known for fuel economy. I generally get 12-14 out of bubba. 4.9/c6 3.08 rear gear but i do have 32,11.50 tires.

My dolly gets better at 15-16 on stock tires and height but thats 5.0/aod with 3.73 gears

I have been out of town almost two weeks now and yes its adding up at 3.25 per.

Im planning a test “build” with mpg in mind. I just picked up an 83f100 to do it and will post as i go. Im going to try.

I would rather try and fail than fail to try”

[85lebaront2]

Darth always got a pretty solid 10 except pulling the 10K 5th wheel, then he would drop to 8. That was C6 and 3.55 rear, Worst was 7.5 towing 65 mph into a 35 mph headwind try to keep up with a German woman (post WWII bride of an Army Air Force GI) in a motorhome. With the EFI and E4OD, even with low compression he got 12.5 going from Exmore VA to Falling Waters WV running 70-75 on the interstates in MD. I am hoping the new engine will pull 14.

[Pete Whitstone]

Oh that's a good point Dane, and I completely forgot that I was told upon purchase that a shift kit had been added to help with an automatic pulling trailers. Not sure if that helps or hurts gas mileage.

I would think a shift kit would not affect mileage at all. All an A/T shift kit does is firm up the shifts, making the shifts quicker and harder. You would think that would not be great for the transmission, but the opposite is true. Just as with a manual clutch, the less time you spend in that "slippage" area, the less the clutch wears. It's no different for an A/T. Getting the shift done quicker and getting out of slippage and back to lockup (not talking torque converter lockup here) limits the wear on the "consumable" clutch surfaces.

The reasons the EOM doesn't build them like that are twofold.

1. Customers prefer cushier shifts to harder shifts, especially in cars.

2. Planned obsolesence.

Oh, and to weigh in on the original subject matter - my 351W/C6 gets pretty bad mileage, right around 10 as far as I can tell. It's a short wheel base stepside that probably doesn't weigh more than about 4000 lbs, if that. So that's pretty bad, but I looked up the original mileage they were getting in 1981, and it was about 10-12. I know my truck is running somewhat rich so I could probably get it up to 12 or so if I worked at it. But I am planning on building a new engine for it, with aftermarket multi-point FI, so I am hoping I can maybe get that up to 13-14. Maybe even 15 on the highway, but that might be too much to hope for since the truck has the aerodynamic qualities of a medium-sized Sears garden shed.