Miles per gallon

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.

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

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.

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.

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...?

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”

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.

delco1946 wrote

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.

Pete Whitstone wrote

the truck has the aerodynamic qualities of a medium-sized Sears garden shed.

These are all excellent points and thoughts. Makes me feel better about where my trucks stands in terms of MPG - prolly should nick-name 'er "Gas Pig"

Pete Whitstone wrote

... since the truck has the aerodynamic qualities of a medium-sized Sears garden shed.

I respectfully disagree.  I think the garden shed is more aerodynamic.  Our trucks have recesses, like around the headlights and turn signals, that catch air.  Garden sheds don't.  Nor do barn doors.

Compare our trucks to the later trucks and you'll see that those recesses are filled in.  So, while I consider the later trucks .... less than pretty, they are more aerodynamic.  

302 C6 all original with a 2.75:1 9" axle with taller than stock 31x10.50-15 KO2 tires.

Winter choke was a little slow for me so I got 12 mpg city due to my less than 5 mile drive to work.

Summer I averaged 14 mpg city.

Highway I average 17 mpg.

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

I think it has a shift kit because the shifts are firm. The torque converter I’m thinking may slip less than stock. About the best you can do without a lockup converter.

My Bronco is pretty consistent at 15 MPG on the highway. My Xterra does a little better, but it doesn’t have the grunt!

I feel like there must be more efficiency to be had although i have not proven it yet. However the benchmarks i have are many and not reassuring.
E150 5.0/aode and 3.31 gear 13 in town, 16 hwy
E350 5.8/ e4od and 3.50 gear 10/13
Expedition 5.4/4r75w 4wd 3.73 gear 12/15
Excursion 6.8/4r100 4wd with 3.73 12/17
These are some from the last five years and the excursion surprised me . About as much as the expedition disapoints .

Rusty_S85 wrote

302 C6 all original with a 2.75:1 9" axle with taller than stock 31x10.50-15 KO2 tires.

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

Switching from a (numerically) lower 2.75:1 to a higher 3.25:1 ratio will hurt gas mileage by making the engine spin more RPM's per mile driven, won't it?

Still on my first cup of coffee this morning so I might be wrong, but I think that's the way that works.

Pete, a bit of that depends on several factors, the basic item is what is called the "sweet spot" where the engine is at it's most efficient rpm. Back when Nixon put in the national 55 mph speed limit, he cost me several miles per gallon on two of my cars I owned at the time.

My 1966 Shelby with the Cobra 289 and sporting dual Holley 4 barrels with a 3.89 gear went from 20 mpg down to 16-17. Sweet spot on that engine was 3500 rpm, almost all of the mechanical advance (no vacuum) was in and I would be running on the primary barrels of the front carburetor (max 232.5 cfm). This was right at 70 mph. Drop to 55, rpm went down to 2750, about 3/4 advance, so there went my efficiency.

The other was my 1963 Oldsmobile Jetfire, it dropped from 24 down to 18, it was a turbocharged 215 ci aluminum V8 and I had removed the enrichment spring from the Rochester RC sidedraft carb. 3.35 gear and the only one of the BOP compacts with 14" wheels as standard equipment. At 70 the manifold gauge would sit just below the center (0 boost or vacuum) maybe 1-2" of vacuum, so the Garret turbo was doing most of the work on filling the cylinders, but mixture was still fairly lean. Drop to 55, vacuum went to about 1/2 scale, and probably less spark advance. Funny thing, the Jetfire distributor had a vacuum advance, so in theory it should have done better with more vacuum.

True, I had not considered that. I wonder if there is a solid correlation between max MPG and VE. I know there is a strong one between VE and torque production, but I don't know it that also means max MPG. Interesting.

Still, I would have to think that C6 is slipping awfully bad to make up a .5:1 swing in axle ratio and still come out better, even with the sweet spot taken into account.

I will say that regardless of the exact set-up, 24 mpg in a non-aerodynamic truck, presumably carbureted, 2 valves per cylinder, no VVT, variable intake tract trickery, etc. is a tall ask from a 5 liter engine. I mean, 2021 Ford truck factory mileage is in the 26-26 range, and that's WITH all the stuff I just mentioned. I realize they are also probably a lot heavier, but still.

I hope Rusty can hit 24, but I'm skeptical.

 

Pete Whitstone wrote

Rusty_S85 wrote

302 C6 all original with a 2.75:1 9" axle with taller than stock 31x10.50-15 KO2 tires.

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

Switching from a (numerically) lower 2.75:1 to a higher 3.25:1 ratio will hurt gas mileage by making the engine spin more RPM's per mile driven, won't it?

Still on my first cup of coffee this morning so I might be wrong, but I think that's the way that works.

Yes and no.  If nothing else is changed it will hurt gas milage by increasing engine rpm at a given speed.

In my case it wont because I went from a 28" tall tire to a 31x10.50-15 tire which is supposed to be 31" tall but is really 29" tall.  That extra inch in heigth puts my effective gear ratio from 2.75:1 closer to 2.50:1 - 2.55:1 which is lugging my engine down.  Like wise my C6 has excessive slip right now cause I should be turning around 2,400 rpm at 75 mph but right now I turn 2,750 rpm at 75 mph which increases fuel consumption as well.  Also the engine I am building the truckmax roller cam I have has a listed cruise rpm of 2,400 - 3,000 rpm for ideal highway speed.

The math I did shows with 3.25:1 axle ratio would put me around 2.90:1 effective ratio when accounting for taller tires.  The C6 I am going to have built by Broader Performance should be around 3% - 5% slip when coupled with the Hughs XTM I believe it is tow master converter that multiplys torque more, flows more fluid, and has less slip than a standard converter.  With this taken into account if I am in the 3% - 5% slip range with a 3.25:1 axle ratio it will put my 75 mph speed right at 2,750 rpm and 65 mph right at 2,500 rpm.  If my slip is more which I wont know till I do real world testing with the transmission I figured a 3.00:1 would put my effective axle ratio around 2.70:1 and between 5% and 10% slip I should have 75 mph at 2,750 rpm.

When making as many changes as I am to my truck you have to do it in steps and make it work together.  Why I might have to get Adam to redo the tune on my sniper once I get the transmission and axle ratio change done as it may want a different fuel table than what he comes up with me with my current setup.  But my goal is to try and keep 65 and 75 mph with in the narrow 2,400 - 3,000 rpm cruise range that is listed for my camshaft as the roads I drive are 60 - 75 mph but I typically go 65 and 75 mph.

Rusty_S85 wrote

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

Rusty, I wish you all the luck in the world, but I have a hard time imagining a 302 running 3000 RPM with a C6 and 31" tires hitting 24 mpg.

Even if you consider the 1987-1996 trucks, with speed density or later on MAF, with 5spd's and 3.08 gears, most of those trucks could barely even hit 20mpg on the highway. They surely wouldn't do in 4th gear at 1:1 ratio like a C6. I can see the Holley Sniper being better technology...sort of...but it's still throttle body EFI, and not multiport EFI.

To throw my old truck into the MPG mix for the OP, it was an '84 2wd Flareside with a mild built 302, a 1991 M5OD 5spd trans, and original 3.08 rear diff. No AC, and no other options really. Truck weighed 3500 lbs. I drove it on the highway quite a bit, and the best I ever got with it was around 18.5mpg...maybe it kissed 19mpg one time, but I'm not sure...my memory is getting fuzzy.

That was at around 2000 RPM in OD, running between 60-65 mph. My AFR was usually a little over 14:1 when I was cruising easily. I did try some highway trips at faster speeds and higher RPM's, and depending on the day it never really hurt my mpg too much...sometimes 1 or 2mpg less, but I think the wind drag negated any efficiency I might have gained with the higher RPM's.

I never did try running the truck up in the 2500-3000 RPM range in 4th gear on the highway to see if the MPG would improve. Maybe that's the answer to getting better mileage...higher RPM's and lower gears...I've never thought to try it.