Every car has a 'sweet spot' for MPG, after which the increases in speed cause your MPG to drop like a rock. I'm not sure what the bird's peak MPG efficiency speed is, but it's likely around 55 MPH. At 75 MPH, you're getting about 15% less fuel economy than you are at 60. At 80, that would be even worse. So with a little math, at 19 MPG running 80 MPH, theoretically you'd be getting 22 or better MPG at 60 or MPH. That goes right in with my MPG; I took a 240 mile round trip this Sunday, 90% highway and got 21.9 MPG with an average speed of about 65 (alternating between 60 and 70 on different highways). I also have been getting 15-18 MPG city lately. I expect that to jump up to the mid-high 20s and low 20s, respectively, when the warm air returns. And I'm running the super cheap brandless 10W oil and 87 octane.Something is definitely wrong here.
I recently did three trips in the last week that were about 90% highway each (NY to DC - 270 miles, DC to NJ - 230 miles, and NJ to DC - 210 miles).
Speed was an average 75-80 MPH. I run 89 octane, and despite it being winter gas, my MPG for all three trips was less than 20. About 19 and change.
That was with a change to clean oil (Motorcraft 5W30) and a Seafoam treatment right before the NY-DC trip.
My city mileage (I've been keeping track for the last year or two) is in the low 14s. Even with summer gas, my highway mileage (I took a road trip to Pennsylvania and back) was sub-20.