F150 MPG Blues

[Noahfreak]

Originally Posted by Frank Lee

If you want fuel economy you're going to have to slow it down to 55 and/or seriously aeromod it.

When you say fuel economy, that's a relative term. I'd be happy to get above 15 mpg right now, because my best tank so far was about 14, and that was all highway miles at 60mph to keep the rpm below 2000. One of my recent tanks was about 9 mpg, which is totally unacceptable. On long trips I hope to get in the neighborhood of 18-20 mpg, which I think is still possible at 65 mph considering that my father's old '77 F-250 4x4 with the 4-speed, crew cab and short bed did 21 mpg at 65 mph.

That's with heavier running gear, heavier curb weight, loaded with about 2000 lbs. of payload, no overdrive, higher ride height and a bigger displacement carbureted engine (351M) along with old-school heavy weight non-synthetic gear oil and taller/wider mud/snow tires. This was no spread-bore 4 barrel Motorcraft carb either, it was a 2-barrel that was quite a bit worse for efficiency. That truck was not the model for high mpg, but if we saw those kinds of numbers with that setup, I can get those or better with this one. I think my expectations are relatively modest considering that it's fuel injected, smaller displacement, much lighter, lighter running gear, is 2-wheel drive and has a lower ride height.

As far as aero mods go, I do plan on at least installing a bigger air dam, and maybe more depending on what that yields. I intend on adding a vacuum gauge to mod the nut behind the wheel as well. In addition, I bought a scanner tool to be sure that all of my engine sensors are reading within their specified ranges.

That said, my first concern is utility. This is a work truck that I don't drive every day, so that's the priority. If anything else interferes with that, it's gotta go. I chose the 300 because I've seen them run for 30K miles with a loud knock in the engine and no oil changes. I've also seen them run 300k miles with no bottom end or head work needed, the rest of the truck rusted in half around it, on its 4th c4 transmission. I want that kind of reliability, and unless you go with the light duty Japanese trucks from that era, you can't beat the setup that I'm building.

I owned a couple of old Nissans as well, and their mileage wasn't much better considering the gearing they needed with those smaller engines to keep the truck moving on the highway. I also like that this engine has peak torque, 265 lb-ft. at 2000 rpm, which is what you need when you're pulling a lot of weight. I don't want to have to rev a 302 to 2800 rpms just to get a trailer moving, and I don't want to go bigger displacement due to economy concerns.

[Frank Lee]

...which I think is still possible at 65 mph considering that my father's old '77 F-250 4x4 with the 4-speed, crew cab and short bed did 21 mpg at 65 mph.

Once upon a time I did an exhaustive search of the EM garage for full-size V8 pickup mpgs. IIRC nobody was able to average even 20. So color me hiiiiiiigggggghhhhhhlllllllyyyyy skeptical of that old beast getting 21 at that speed.

[Noahfreak]

Be "hiiiiiiigggggghhhhhhlllllllyyyyy" skeptical all you want Frank Lee, that doesn't mean it didn't happen.

Besides, I wasn't referring to its average mpg, but a specific trip that was 1000+ miles, meaning that several tankfulls were all highway miles within one day of driving. The truck was already hot and there were no rest stops between fillups. It had a cap on the back, which probably helped the aero some. All of the oils had just been changed, the carburetor had just been rebuilt and the ignition was all new as well. The engine and running gear was well broken in and had only about 50K miles on it. Most of the terrain was flat or had very little grade. I'm sure our average speed was lower than 65 mph (about 10 over the posted speed limit) due to traffic and speed zones, but there were several long stretches where that speed was run. Also keep in mind that this was done back before ethanol was introduced into unleaded fuel, which probably boosted the mileage some compared to today's fuels.

The average mpg on that truck was closer to 10 or 12 mpg because it was almost never run on the highway. It was also almost always towing a trailer or being used to plow snow or run on dirt trails that required sub-5 mph driving on trips that took less than 20 minutes. That's why at 90K miles it was completely rusted to the ground and we junked it.

We didn't monitor mpg regularly in daily driving though, because gas was about $0.98 a gallon at the time, and 20 bucks would fill the tank, so I can't say for sure what it was at any other time. We were just curious on that particular trip, and we got that mileage in both directions.

That said, the near-perfect conditions for mileage described above were very similar to a tank that I ran through my '88 recently (except that my engine has a little over 1000 miles on it since the rebuild, so it's not broken in) up to LA from San Diego, and I only got about 13 mpg with it on that trip, and I only ran it at 60, so I know something was very wrong. I filled up just before I left and topped it back off when I got back, so that's a pretty accurate highway mileage for the truck.

The transmission swap was a good start considering how much more power it seems to have now. I'll get to the sensors soon. I'm confident I can hit 20 mpg on the highway, running 65 with this truck, especially with aero mods.

[Noahfreak]

Originally Posted by Frank Lee

Once upon a time I did an exhaustive search of the EM garage for full-size V8 pickup mpgs. IIRC nobody was able to average even 20. So color me hiiiiiiigggggghhhhhhlllllllyyyyy skeptical of that old beast getting 21 at that speed.

There's also no 351M F250's on here, so there's nothing to compare that to, apples to apples.

[ksa8907]

If you want better mileage on the highway you really need to slow the engine down more. Peak torque at 2k and you're running down the road at just under that? I'd look for better rear gears.

[Noahfreak]

Originally Posted by ksa8907

If you want better mileage on the highway you really need to slow the engine down more. Peak torque at 2k and you're running down the road at just under that? I'd look for better rear gears.

Thanks for the suggestion. Yeah, that might be something I'll do down the road. I want to see where I'm at first with the new transmission though. There are lots of options for the 8.8 rear end considering that they come in Mustangs and Crown Vics, so I should be able to pick up some take-offs pretty cheap. Is there a specific target, percentage-wise, under the peak torque number that I should be shooting for? I've heard that these 4.9L motors get worse if you go too low in the RPM range too, so I want to avoid that if possible.

[trooper Tdiesel]

is it a S\B reg cab or a L\B extra cab, or one of the other combos
that makes a big difference in the empty LBs and amount of power needed to move it..

what tire size are you running on the F150
something around 235/75/15 was the OE size...


i see them swapped to 31"s all the time and it affects them a fare amount..

2.73/3.08/3.27/3.55/3.73/4.11/4.55

where made for the F-150s in the 1980 to 95ish models
the 3.27 ratio may only be a 90s thing..and the LSDs only fit the 3.08 and up diffs.

under stand the more you rigg it for light loads on the open road, the more it will hurt your loaded and or city mpg...
as the saying goes there's no free lunch when it come's to gearing...

for road mpg.....
0.01c down size the tires and compensate with the final ratio....

i figure a 2.73 with a 4sp would be about the best
though ether version of the ZFs driven as a 4sp would work as well.
with the deep 1st it would be ok, though they have a very tall reverse...

a direct drive gear with a 2.73 makes for about the same as the 3.55 with the O\D
mixing the 2.73 with the 25% O\D and 215/75/15 makes for about 1,500 at 60 VERY MUCH A FLAT LAND EMPTY GEAR.


the 300s are old enuf there computers are not specd for E10 there all pre OBD-II 1996
if you can find E0 gas that's with in 0.30 cents of E10 it would be brake even or lower cost per mile..to run E0 gas..

edit..gearing list of all the different ZFs.
ZF Manual / Standard Transmission Identification Parts and Diagrams

[user removed]

My 1992 would do 20 MPG on the highway, probably 17-18 around here locally. It had the Mazda tranny which was a .80 OD and the 2.73 rear axle. Only had 90k miles when I bought it. Sold it later and eventually got my Ranger which has the Mazda tranny and a 3.45 rear axle. Lightest duty setup they sold. Averaging 32 in the Ranger, basically the same driving environment.

regards
Mech

[Noahfreak]

Originally Posted by trooper Tdiesel

is it a S\B reg cab or a L\B extra cab, or one of the other combos

what tire size are you running on the F150
something around 235/75/15 was the OE size...


under stand the more you rigg it for light loads on the open road, the more it will hurt your loaded and or city mpg...
as the saying goes there's no free lunch when it come's to gearing...

for road mpg.....
0.01c down size the tires and compensate with the final ratio....

ZF Manual / Standard Transmission Identification Parts and Diagrams

Thanks for the info Trooper Tdiesel

It's an extended cab long bed.

The current tire size is the 31x10.50x15 that you mentioned people changing them to. I think they came stock on the 4x4's and broncos. I plan to return to the OE size or at least something with a little better rolling resistance than these offroading tires that are on it now.

Right now the rear gears are the stock 3.55's and I have the option to switch to whatever's available. The ZF S5-42 that I just swapped in is the wide ratio with the 0.76 overdrive gear. I think the most I'd want to go right now would be around a 3.08 if I keep the current tires.

Sorry for being a newb, but what do you mean by 0.01c on the tires? I'm assuming you mean just go shorter/narrower?

As far as I know, E0 gas isn't available anywhere in California, so that's not an option for me. Even if it was available, it would cost me more to drive to some distant station than I'd save in mileage. I've noticed that there are tuning chips for the 302 version of the F150 to improve mileage for E10 but I haven't been able to find anything for the 300. Anyone know of one, or a way to hack into this thing myself and tune it?

[Noahfreak]

Hi Everyone,

I just wanted to give a thread update for the mods I've been working on. I recently made a cross-country trip so I did a bunch of mods to prepare for it:

-Reduced tire size from 31" tires (~265/75r15) to 235/75r15's. They're Wranglers, so not the best, but much better than the off road 265s. This lowered the ride height some too.
-Switched from 3.55:1 to 3.08:1 rear gear ratio, filled with synthetic oil, and put a brand new driveshaft carrier bearing on.
-Installed a vacuum gauge to monitor my throttle position, which helps a lot! This engine is super quiet, so I have no idea what the engine is doing without looking at the tach (or now the vacuum gauge).
-Installed a new fan clutch to make sure that's working properly.

On the dead head portion of the 5300 mile round trip, I averaged about 14.5 mpg for the first few fill ups. While I was running through mountains for much of this, this seemed a little low to me considering I was getting that around town the week before I left. Since it was 75 degrees when I left San Diego, and the temperature dropped as soon as I got into Arizona, I finally figured out by the time the temp was in the single digits (polar vortex) that I should recheck the psi on the tires, which were set at 35 when I left. They had dropped over 10 psi by the time I was in Oklahoma! I pumped them back up and immediately saw a huge jump in mpg. The last day of the trip I averaged 18.7 mpg on the final leg! That puts me really close to my 20 mpg target for highway mileage.
Another interesting thing of note was that the amount of vacuum that the engine would pull at elevation was significantly lower than at sea level. It dropped to the mid-teens at the highest elevations, while it sits at 23 inches of mercury at sea level.
On the way back, I had about an extra 4500 lbs on the truck, including an enclosed trailer which really screwed up my aerodynamics (no cap on the truck), so my average dropped back down to 12 mpg for the rest of the trip. I had hoped for a little better than that, but I think that's pretty decent since it's better than the unladen mpg I got when I first purchased the truck, and the majority of my trip was in sub-freezing temps.
Since I got back in February, I've made a few more mods:

-The new engine has about 10k on it finally, so I just switched to full synthetic 10W-30.
-Installed a K&N 57-2503 FIPK intake kit. I would have made my own, but, California.

People have borrowed it from me several times since then so I haven't had the chance to monitor mileage, but I'll let everyone know if I hit the 20 mpg Holy Grail I'm shooting for on the highway.

Some planned mods:

-Exhaust. I'm planning on a 2.5" home-built setup that will start right after the second cat and will exit in front of the passenger rear tire.
-Thinking about how my aero mods are going to go. I don't want these to draw any negative attention from the cops, so I'll have to take some care to make sure they're aesthetically decent.
-I've also heard that these things respond well to port and polish jobs on the head intake and exhaust bowls. I have an extra head in storage right now, so I may go to town on that as well with a Dremel tool.
-The trip I made to Buffalo was mostly to pick up a Bridgeport J-Head Mill, so now that I have the capability to machine the head, I might also install an adjustable valve train on it with roller rockers and screw-in rocker studs. I don't like that I can't set the valve lash on this engine without buying different length push rods, and the roller rockers should help reduce valve-train mechanical losses. We'll see when I get to that.

Sorry for the long post. A lot has happened since the last time I updated. Let me know what you guys think!