I used krousdb and MetroMPG's marvelous Aerodynamic & rolling resistance calculator - EcoModder.com to compare my 2009 Prius at 55 and 80 and see what the real costs were. I visualized its results
I was comparing my daughter's 80 MPH with my own 55 MPH. This chart shows that both my daughter, at 80 mph, and I, at 55 mph, spent the same energy, 115 watts, to overcome rolling resistance. But at 80 mph Margaret (my daughter) needed to spend over double (452 watts) the force I used (214 watts) to overcome the increased wind resistance. This brought her miles per gallon down to 28 mpg, 42 % below mine at 48 mpg.
Margaret, at 80 mph, would cover 150 miles in about an hour and 50 minutes while it would take me, at 55 mph, 2 hours and 40 minutes. However, to save those 50 minutes Margaret, speeding at 80 mph, burns 2.28 more gallons and adds 44 more pounds of carbon to the atmosphere than I would have at 55 mph.
This is no new news to anyone in this Forum, but it might allow me to teach My daughter to slow down!