On the way from Houston to Dallas yesterday, we encountered a serious downpour that appeared (as Texas rains do) quite suddenly. It was the kind of rain that caused us to slow from 70-75 mph to 50-55 mph and turn our hazard lights on so that we weren't hit from behind by someone not so prudent as to slow down. The water was deep enough that we could feel a drag from the water on the road and the wipers were on their highest setting, barely keeping the windshield acceptably clear. Prior to the rain, we were averaging 300 to 325 Wh/mi on the flat, dry highway going about 70 mph. During the torrential rain, we were averaging about 480 Wh/mi, even after having slowed to 50-55 mph. Data point: we currently have factory 19" tires on the car. After 20,000+ miles, I'm well aware that heavy rain increases power usage, but this was the first time I quantified it. Fortunately, we were out of the rain in about 20 minutes. If the entire drive from Houston to Dallas had been experiencing rain that heavy, it would have been a more challenging drive home.