Stop in for a cup of coffee

Riddle me this.

On the drive thru PA, I averaged 19.2 mpg there, 19.4 mpg back,

But in Ohio and Indiana, I got 15.5 there, 17.0 back?

Running 80 the entire way? Except for the part on the way back on route 30 in PA
Elevation. Uphill from your place to PA and downhill back. It changes more than people think.