Crosswinds are almost always more detrimental to mileage than a direct headwind. The first screenshot below shows a baseline with several arrows explaining much of the graphical detail. This first frame shows No Wind & the Prius getting 50.49 MPG. Next we show a 10 MPH Headwind which drops the mileage down to 43.06. The next frame shows a crosswind from 10 degrees off the right-front of the Prius and the mileage goes further down to 42.28 MPG. Follow the series of frames and watch the very many interesting effects of crosswinds. Notice also that we do not get back the equivalent mileage of a 10 MPH headwind until about 90 degrees. Then notice that the crosswinds coming even from rearward side of the vehicle do not help until they get to about 130 degrees where the 'Ambient Wind' graphics goes from Red to Green. This is called the 30/70 Wind Rule and is explained below by clicking here.