You likely had a tail wind, which is a big factor at higher speeds. But, that's still good. I just did a 1000-miler in my Chevy Cobalt and only got 30.4 MPG.
Aerodynamic drag force is roughly proportional to the square of velocity. So, 80 MPH with a 10 MPH tail wind, your aerodynamic drag force is reduced by 23.4% (70^2/80^2=0.766). But, 80 MPH with a 10 MPH head wind increases the aerodynamic drag force by 26.6% (90^2/80^2=1.266).
I used to have a lot of work in south Texas. Texas is windy, usually coming from the west. Gong out there my MPG was low, and coming back it was high.