I recently bought a 3.5 ecoboost. I have an odd thing happening. I am getting around 16 mpg when on the interstate 70-80 mph but when I am on local roads I can get up to 20 mpg and even in the city still average better than the HWY.

The truck does have 33" mud tires on it but its still the first vehicle I have ever had that got worse gas mileage on the interstate.

anyone else have this issue?

 

How are you determining this phenomenon?

Your truck is obviously broken.

 

The mpg monitor in the dash. I reset it before about a 15 mile trip on the interstate. reset it before the same trip on the hwy. They run parallel from my house to work. and reset it for a while while in town.

 

Perhaps once you reset it, it's actually getting great mileage based on the established highway pattern and hasn't had enough time to adjust while in the city?

If you drove for 30 minutes on the highway then reset and drive in town for 10 minutes it may still be calibrating.

Just a guess for sure. No idea how long either of the 'averages' were calculated.

Don't jinx yourself. Never come back to this thread again and simply drive your city economy vehicle around while the rest of us struggle at good fuel economy daily regardless of where we drive.

 

Do 60-65 on the highway instead, see what happens. And don't underestimate the affect of huge mud tyres.

 

What is your gear ratio? Driving over 70 drops mileage like a rock on Jupiter.

 

15 miles isn't far enough to get a meaningful reading, when you've driven it a few hundred miles get back with us. You burn a LOT of fuel during the 1st mile or so trying to get from 0 up to 70-80 mph. During that 1st mile fuel mileage will be in single digits. But once up to speed you burn much less fuel. It takes lot more than another 14 miles for things to even out to balance the 4-5 mpg you got the 1st mile. After a fill up the 1st 20-30 miles is always low, but on a long road trip it continues to climb during the day.

Anytime you drive a truck over 70 mp fuel mileage takes a big hit. Most cars won't be much different, but with trucks with the aerodynamics of a brick fuel mileage sucks at 70+. Keeping it down to 65 can make a big difference.

Since you're not dealing with wind resistance at slower speeds you're getting better fuel mileage in town

Your 33" tires have both the speedometer and odometer off. When you think you were driving 70-80 you were actually driving 73-83 mph. Which compounds the issue above even more.

Mud tires, especially those wider than stock have much higher rolling resistance. This shows up more at high speeds.

Since you're odometer is off by about 4% you've actually driven farther than you think. If your trip meter says you drove 250 miles you really drove about 260. Take that into consideration when hand calculating fuel mileage

Don't trust the onboard calculations, do it by hand. Sometimes they are accurate, or pretty close. Other times they are way off. Fill up until the pump clicks off, set the trip meter. Drive until you burn up at least 1/2 the tank. Fill up again until the pump clicks off. Look at odometer and multiply miles driven by 1.04. to determine the actual miles driven. Then divide miles driven by the amount of gas you used.

Example: Your odometer says 250. 250X1.04= 260 miles actually driven. Fuel pump says you burned 13.6 gallons. 260/13.6=19.11 mpg.

 

65 seems to be the magic number on my trucks. Then it drops like a rock. These engines don't like high rpms.

 

I would defiantly drive it longer then report back. I would drive it for a couple hundred miles then see the result. If I reset the mpg calculator then drive for 15 mins carefully my truck will tell me I get 15-16 mpg but after a couple hundred miles it gets down to the 12ish range from combined driving and more accurate calculation. I have a 2012 ecoboost with 33" mud tires w/2.5" level and I would absolutely love that gas mileage. My truck gets 12-13 on the highway and 10-12 in the city. And that is me trying to stay out of the turbos as much as possible but with the bigger heavy tires thats harder to do. I would agree 65 is right about where my truck is happy anything over 70 and the mileage takes a dive quickly and the turbos are almost always spooling. I should note my truck is not tuned either.

 

Any time you go over 70-75 mph you're seriously hurting your mpg's. My truck does the best between 45-55; it'll easily hit 23mpg (calculated) on long stretches of 2 lane roads with no stops. Even in the city, as long as you aren't getting stuck at many lights and aren't gunning it from a stop you can do OK. Those large tires are hurting you at highway speeds, try staying closer to 70 and seeing what happens. I've found 73 mph is a happy medium where I can still get 19-20 mpg, but I'm running a stock tire size with a highway tread.