11-22-2017, 02:14 PM
Join Date: Apr 2016
13MPG seems quite low for highway. I don't have an '04 but my '98 4.0 SOHC gets around 20 Hwy. It would be 1-2MPG higher if I drove longer distances. Keep in mind that it has to get up to temperature for optimal fuel economy so if a significant % of your hwy miles are before it has warmed up, your MPG will be low like that.
They're rated for 21MPG but that is warmed up the entire time. Now that mine is older, not a daily driver but rather shorter trips around town, I get horrible stop and go MPG like under 10MPG, even under 8MPG with few stretches of driving more than a half mile between stopping at a light or sign, and not getting up to temperature much of the time.
It has stock tires and the higher 3.73 rear differential which both help, and I try not to go much above 65MPH (closer to 55MPH would help more). What is your steady speed engine RPM on the highway and at what speed?
Is your check engine light on? You could hook up a scan tool even if it isn't, one capable of looking at live data to see if it's running rich (fuel trim numbers). Sometimes as an O2 sensor(s) get older, they get less accurate for a while before they get bad enough to set an OBDII trouble code.
A slight (enough to go unnoticed) oil consumption, or a head or intake manifold leak causing coolant into the cylinders, can start to clog the catalytic converter reducing MPG. A cylinder misfiring for one of various reasons, just not often enough to set a trouble code can also reduce MPG, as could bad engine wear if it's not been maintained with oil changes as needed - yet that and misfires, usually wouldn't run well enough to be called "really smooth".
I assume you've checked the regular maintenance items like clean air filter, enough air in tires, spark plugs not too far past their 100K mi recommended replacement (though at this age even if you're not past 100K mi you might want to replace them anyway, time also takes its toll).
You might also check the condition of your brakes, that they aren't dragging, though if that is happening, soon enough you should find that they're worn out. You could also do a cylinder compression test to see what state that's in.
These vehicles are Bricks On Wheels and it's easy to get no better than 15MPG except when driving over 10 miles at a time, but you wrote highway so there may be some differences between what I'm talking about and what you are.
If you have larger diameter than stock tires, not only can they reduce fuel economy a little, but also make the vehicle travel faster than measured so your real fuel economy is higher than the odometer shows. Aggressive all terrain or especially mudder tires will also reduce economy. That noise and vibration they make is an efficiency loss.
Ultimately my point is, you could have a specific problem, or it could be your driving environment/habits, or a combination of multiple little things robbing a MPG each.
Please post your vehicle year/model/engine/(n)WD in help topics.
'98 Explorer XLT 4L SOHC 4WD
'14 Explorer XLT 3.5L NA AWD