Comparison in the Prius - Mrs A and I had to travel down south recently and the start and end points were both at pretty much sea level - we used the same route on both occasions. On the way down I kept a high-ish average speed as we were in a hurry to do the trip in a day before some forecast gales struck the UK. Average (on the on-board computer) was only 42 MPG.
On the way back Mrs A took it easier, the average being 10 MPH lower (we filled up at both ends and reset the trip) - average was 49.
Actual speeds withheld to preserve licence point status 