Got a question for other users of the forum and I applogize in advance if this is a bit off topic or has been asked in the past.

I've got a '01 S'Crew 4x2 I picked up recently that has been lightly used. It's a 4.6L V8 automatic with 3.55 gearing. What I have seen regarding gas mileage has been odd, at least in comparison to my '99 S'Cab 4x2 with 4.6L V8 automatic. My S'Crew has 255/70/16s wheels where my S'Cab has 235/70/16s, for what's its worth.


In my city driving, I have been averaging 16-16.5 mpg for about 10 tanks now. This hasn't surprised me, it's similar to what my 99 got(16-18mpg) and certainly within what Ford specs.
What is odd, at least to me, is on multiple long distance(100 miles+) interstate only trips, I also average 16 mpg. This is with a lightly loaded truck, full tank of gas, some times with headwind, other times with tailwind,and over flat terrain. I have tested with cruise control and without cruise control; this is averaging between 70-75 mph, right around 2000 rpm. My other truck gets between 20 and 24mpg on the same route over multiple trips.
I measure mileage from full tank to full tank and reset the trip odometer to track the mileage.

My truck is stock in all regards. After I bought it, I replaced the fuel filter, changed the oil and oil filter and inspected the spark plugs and ATF fluid and level. On the off chance that there was water in the tank or some other problem with fuel, I've used some fuel treatments to no effect.

I always fill up with 87 octane unleaded, and have tried different stations in different cities to see if there is some noticable variance to my mpg.

The engine has plenty of power and acceleration. I've not noticed any hesitation during accelerations nor roughness at idle. I haven't found any odd sounds while driving or while parked. I'm not a lead foot and keep the RPM low.

My most recent and as of yet incompletely verified experiment is dropping in a K&N air filter to see if the mpg changes noticably, though I doubt it will effect a difference between the city and interstate mpg.

Anyway, after reading through several years worth of mileage posts and FAQs, thought I'd post my situation and see if any one else has seen this behavior before and what else could be causing it. And as the old expresion says "your mileage may vary", so I understand each and every truck is a bit different.

Thanks,

 

You're doing better than my '97 v6 w/3.55 rr.I get 15 city /15 highway.Can't explain it.just accept it

 

You're really doing pretty good. My '01 Screw 5.4l 4X4 gets about 12 MPG around town and used to get only 13 on the highway. I have made a few modifications and my highway MPG has gone up to 15.5 but my city has stayed 12.

 

An update:

Based on some advice from some friends of mine and some independent thinking; went ahead and cleaned off the MAF sensors and pulled the battery cable to allow the engine timings to reset.

I would say that the K&N filter has helped. Since I installed it, my mileage has gone up to 18 mpg from 16 mpg, in mixed city/highway driving. If my calculations are accurate and if I can reproduce the mileage, then hopefullly I'm back to the Ford norm for my particular S'Crew's mileage on the highway. Time will tell.

 

My 01' screw 4x2, 5.4. 3.55 gets 16-16.5mpg every tankful (after the first two) since it was new. It doesn't matter if it's all city, all highway, or mixed. I feel like overall that's pretty good. I do have a light foot though. I bet you might notice a bigger difference between the two if you had a heavy foot. That would tend to hurt city mileage the most.
By the way, I've added a Volant intake, Poweraid intake spacer, and a Magnaflow muffler. One at a time. Power and driveability have really improved but the mileage NEVER changed.