So my truck had been suffering from the dreaded failed fuel pump return line check valve, which was filling up the midship tank when I ran off the rear tank. I drove the truck over 1,000 miles in this way since I got it, and averaged between 10 and 13mpg, without accounting for fuel filling up in the midship tank.

Finally replaced the midship fuel pump, which wasn't working reliably for me to even run off the midship tank (had to resort to pumping the gas out periodically back into the rear tank). Just took my first hundred mile journey and the truck returned an honest to god 20 miles per gallon, averaging 65mph on the highway and with maybe 10 miles of stop & go. Is this a freak of nature, or have other folks gotten mileage this good, consistently, with their trucks?

It's a '97 F-250HD, extended cab long bed, E4OD, 2wd with 3.55 gears.

 

I think you may have mis-calculated something......

 

No sir, double checked my figures and filled tank to the brim before starting the trip meter and on the last fill. GPS verified miles driven.


edit; I'm not really EXPECTING this kind of mileage, I'm ecstatic with anything north of 15.

 

Even if it were pure highway miles, 20mpg is unheard of in the 5.8L. More like 13 maybe 14 mpg even with a strong tailwind, downhill coasting, while numerous hamsters are running on the treadmill, and granny is outside pushing. Something makes me think you had a bit more fuel in the tank(s) then the gauge was indicating.

 

My 351 swapped f150 zf just turned 20 mpg highway i recalculated it 3 times stll cant believe it but it did

 

Well, I have another 100 miles to drive tomorrow so I guess we'll see if it's a freak or if I can repeat it.

 

I got 18 on the highway once with my 5.8. However, it was calculated using Canadian gallons. My overall mileage tends to hover around 12 to 13 Canadian MPG.
I hope you continue to get excellent mileage from your truck!

 

Not hard to believe. I've gotten similar mpg multiple times with both ford and gm pickups of this generation.

 

1 trip is no way to measure mpg, and you cant possibly know exactly how many gallons you used. take note of your starting mileage, fill up 10 times, record exactly how many gallons you put in on each fill, and then do your calculation when your tanks are near empty from the last fill.

don't forget to do at least half of your driving in stop-and-go rush hour city driving.

 

MPG FYI:

I have a 96 351 4x4 that has the same check valve issue. So I only run off the front tank. In that front tank the sending unit does not work, so I have to go off of the odometer.

For the last 6 months I have been running the tank to the last drop (using the rear tank as back up) and getting on average 245 miles on a 16 gal tank.

So I am getting a combined 15.3 mpg.

I have taken two highway-only trips with the same truck getting 270 (two trip average) miles per 16gal tank.

Highway average of 16.8.

This is with a K & N Cold air on the truck, I gained 10 to 15 miles per tank over stock breather.

I plan to write a MPG post once I get dual exhaust, electric fan, and a throttle body spacer put on.

The quest for MPG continues.

 

Was that an open air filter, a drop in panel filter, or something else? I'm gonna have to call BS on your claim if it the filter was open to hot under hood air.

Don't waste you money on a TB spacer it can't possibly do anything positive, but do post up whatever data you have.

A good running 5.8 driven conservatively can deliver mid teens and into the high teens strictly highways, best I was ever able to do was 18mpg but that was at 70mph so there was more potential there.

 

I had one trip of 20mpg, stock tires, cool air temps, summer fuel, slight tailwind... 180 miles and 8.? gallons until fuel spilled out (I didn't believe it was full). That was once in over 10 years owning the truck. Normally with that setup I could get 17 on the highway regularly. Now I have tall tires, flat bed, heavier right foot, and 15 is best I see, with 13-14 more normal for highway. So, sure, I believe 20mpg but not as an average over long term. It is a good indication you're on top of maintenance, so keep that up!

Truck is a 96' F150, 5.8L/E4OD/3.55:1/SCSB/4x4. K&N drop in filter.

 

Yeah its an open air:

K&N 57-2510-1 - 57 Series FIPK, Performance Intake Kit

The link has dyno specs for power gains. 302 and 460 only, but we can assume that 351 would be leaning towards the 302 gains.

Puts off a nice whistle too

I am just operating under the concept of more air in, more air out. I dont imagine the denser charge off that spacer would get me much alone, but with the compound of filter/spacer/shorties/duals it would help. Especially since I am driving 500ish miles per week.

 

In your first post you called it a cold air intake. It doesn't look like it is. If it's an open element filter under the hood you replaced the factory cold air intake with a hot air intake. Typically that costs you more than anything else you might have gained since there's more "air" in cold air (it's denser).

 

I agree with you that its an open air, I mis-spoke. However it utilizes the factory scoop once you detach from the box to force cold air into the filter.

Grant it, since its not sealed, yes there is going to be a large amount of hotter air as well. But since the factory box is so restrictive, youre still getting gains

I would venture to say that the new filter has 3 or 4 times the surface area to intake air.

Volume vs Density. In this case volume won

BTW: pre filter kit I was averaging 230 miles per tank, and now 245. The only reason I know this is because the sending unit is bad and I drive off mileage.