F150 MPG Blues
 

Hi Everyone,
I'm new to the forum and already enjoying the wealth of information I've stumbled across. A couple years ago I bought an '88 F150 2wd with the 4.9L I6, Mazda 5-speed and 3.55 rear gears thinking that I'd be able to get decent gas mileage with it. Well, I was wrong, at least so far. My best tank so far has been 15mpg and that was everything I could do that I know of to conserve fuel.
Since getting the truck, I proceeded to mechanically restore it to stock condition before making any eco-type improvements, so I've rebuilt the engine, rear-end (added a limited slip 'cause it only cost another $50 or so compared to rebuilding my original carrier) and I'm about to swap transmissions.
The truck has all brand new ignition, new injectors and as far as I know, the engine is running as it was designed to. Unfortunately, that still means that it's extremely bad on fuel and that it isn't all that powerful.
I have decided to switch from the M5R2 Mazda 5-speed to a ZF wide ratio 5-speed rather than rebuild the current transmission. The ZF has a higher numerical 1st gear for easy starts, especially loaded, and a numerically lower overdrive, hopefully getting my engine below 2000 rpm while cruising at 65 mph. It's also much heavier-duty, so hopefully it will last longer than the current one did.
I decided on all of that before discovering this forum, but man I'm glad I found you guys. My thinking was correct on the transmission mod and making sure the engine is running right, but there's so much more to the puzzle than I originally knew to maximize mpg. There's already a vacuum gauge in the mail for this truck, and I got another one for my 1994 Saturn SC2 even though that thing is already amazing on fuel. I also have a Ford-specific OBDI scan gauge for the truck on the way as well. I think some of the old sensors might be sending faulty signals to the computer. The check engine light is not on, but apparently this could still mean some sensors are out of whack.
There's no way this thing should be so bad on fuel the way I've been driving it. My father's old '77 F250 4x4 crew cab with the 4-speed non-overdrive manual behind the 351M motor would beat this by about 5 mpg, and that's with a carburetor, so there's definitely some improvement to be had.
As far as the Saturn goes, I added a less restrictive intake, hotter ignition coil packs (so I can gap the plugs a little wider), put in new injectors and generally just keep the car well maintained. When I bought it, it was averaging about 23 mpg city and about 26 mpg highway, which is terrible for such a small aerodynamic car. With the cheap mods that I did, it bumped it up to an average of 28 mpg city and a max of about 42 mpg highway. I average in the high 30s on long trips usually. I'm hoping that by adding the vacuum gauge I'll be able to get it into the 30s in the city with daily driving, and bump my best 42 mpg up into the high 40s or maybe even 50 mpg. For the truck, I'd be ecstatic if I could get into the 20s, which I'm sure with some dedicated effort, I will exceed.
I think with the help of these extremely knowledgeable forum members, I'm already well on my way to making these vehicles into the fuel sippers that I hope they can be. Thanks already to those who have provided knowledge. I can't emphasize enough how beneficial that is to the rest of us.

Welcome to ecomodder - and probably a new addiction ;)

Consider Me Addicted
 

Yeah, I'm obviously already addicted to cars, so this will just be a new aspect to obsess over. My nerdiness shows in how excited I am to get started on my transmission swap tomorrow or Tuesday. I hope to video it and post the job on YouTube. Will add a link here if I am able to do so.

I had a 92 F150 with the Mazda tranny and the 300 six. Best mileage I ever saw was 20 on a pure highway trip at 65-70 MPH. My solution was to get a 97 Ranger 2.3 5 speed and a 3.45 rear end. Best tank so far in the Ranger was over 33 MPG. For some reason the old 300 inline just is not that good on mileage and I have heard the carbed ones could do better, but I think the 5.0 has better heat distribution and may actually be the better choice. My solution was to drive my 150 very little, later I sold it.

regards
Mech

Good luck. My dad brought down his F-150, which I sometimes drive instead of Bacon, but I really try to leave it parked.

I had a 1978 F150 with the 300 an automatic and a bad one barrel Carb. This truck also had full running boards and I always got better than 17 MPG. Check your sensors.

Welcome! Looking forward to seeing your work :)

First Mod Done!
 

I got the ZF transmission installed! I had to make a custom-length driveshaft and a couple of things didn't exactly go smoothly, but the transmission is in and it works great. The creeper 1st gear hits 2000rpm at 7mph which is ridiculous, but just fine for me. It's a work truck, not a race car. I'd rather have the added towing capacity than acceleration capability.

The overdrive 5th gear puts me just under 2000rpm at 65mph as well, which is exactly what I was looking for. The old transmission was at about 2150, which apparently is the point at which the old 300 i6 likes to just guzzle the fuel.

I have a couple of pleasant surprises with the swap too. It has gobs more power and torque in any gear at any speed or engine rpm. I used to be able to floor the throttle and not feel any extra pull from the motor. Now, just a slight bit of throttle tip in and you get that seat-of-your-pants torque feel. I think the bearings were so bad on the R2 that the parasitic loss of power was exponentially greater than it is now. I'm sure the synthetic Dexron VI that I put in doesn't hurt either. I'm going to hold off on my vacuum gauge for a tank to see what sort of mpg the tranny swap alone yields. I'm guessing I'll get a few more mpg's even if I beat on it, considering how much peppier it feels.

There also used to be a shimmy at about 20-30mph that I thought might be tires, or even worse, the rear gears. Turns out it was the M5R2 or my old driveshaft front half, because the shimmy is gone! While testing out the driveshaft that I shortened, I took it all the way up to 70 after starting out on some slower streets, and it's smooth as glass all the way through the speed range now. Overall, I'm very happy so far with the results.

YouTube Video of First Mod
 

I chronicled the whole process of the Mazda M5R2 5-speed to ZF S5-42 5-speed swap on video. I have the three video playlist up on YouTube here:
https://www.youtube.com/playlist?lis...ture=c4-feed-u

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.

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.

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.

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

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.

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

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

Thanks for the info Trooper Tdiesel :thumbup:

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?

Update on Mods
 

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!

Hey, kinda late to the party, but since I like that 300ci straight six, and that it has EFI, I think you can get that 20 mpg out of it. I would either a) get an adjustable timing set or move the stock set around until you get maximum cranking pressure with the stock cam or b) get a proper camshaft that will boost the torque without freaking out the computer ( Clifford Research, Crane, Comp, somebody should have something that doesn't require a ton of spring pressure and keep her smooth and civil ). Next would be a header ( I don't know if it's true but they can be a pain to install with EFI but worth it ), high flow cats, going to a quiet, high flow Donaldson muffler ( straight through, cheap ), 2.5" pipes from header to cats ( I believe it's a split dual style ), merge to a large diameter single pipe to muffler. Tuning is everything at this point, find out what the A/F ratio is doing, start doing timing adjustments ( I guess you're running 87 octane, so I know with the chamber in that head you can't get too frisky without her rattling ). Wonder if there's a way to tune that EEC IV?:confused: Add an MSD 6 box and coil.

Wish the EFI was a sequential unit, I think it's batch fire, but I don't know if it would be any consequence. You need a wideband though. I believe that the trucks are tuned on the fat side so that the sucker doesn't melt down when she's working.