Took a highway trip over to Half Moon Bay, first time with 4 big people in the car, about 830 lbs total, started 43.X MPG on MFD. After 30 miles highway and some hill climbing, was at 44 MPG. I was a little surprised. That's the least economy I've ever clocked on the freeway. Came back with bit lighter load, finished up day at 48.3 MPG. 39 psi fr/ 37 rear in tires. I can see where the 2010's 1.8L ICE would help economy for max loads at highway speeds.
A few hundred feet in elevation difference or a 5 mph headwind can knock off 5 mpg easy. If you had a head wind on the way there, most likely you had a tail wind onthe way back. That would explain your results. I have never seen much difference in FE due to weight. Maybe 1 mpg at most.
