I was driving on the expressway this afternoon when I was hit by a big downpour (tropical storm) and the mpg immediately suffered. Lights on, AC on, wipers on, plus the extra rolling resistance of water on the pavement...big difference.. I was not expecting it. :smow:
You'll find more discussion here: http://priuschat.com/forums/gen-ii-prius-fuel-economy/43329-how-does-rain-affect-mileage.html