I went to meet friends for lunch, there was a very heavy downpour, ie: 3 inches per hour rate. I went to enter the highway, and not heavy on the accelerator either, my RPM showed just over 2800 on my scan gauge, I could feel/hear the engine revving, but my speed was not increasing at a rate I thought it should or does in dry weather. I backed off the pedal a bit, to 1700 RPM, and I seemed to be maintaining the acceleration rate I had with the higher RPM. I can surmise it might be the weight, or lack thereof, of the C. I run about 36 PSI in the tires. for the record, when I left the house it was just a steady rain, then when I made the turn to hit the highway the rain opened up into a torrential downpour. There was no option where I could not stop or pull over. a little down the road it lightened up a lot, then rained even harder, luckily it was at my exit, crossed the highway and it turned to a drizzle. crazy southeast Texas tropical weather.
If there was even a shallow amount of standing water on the road then it can take quite a lot of power to accelerate through it. And at that kind of rainfall rate it wouldn't even have to be standing, there'd probably be a significant amount even if it was draining off the crown of the road. That would be my guess as to what you experienced.
I agree with the two posters above and similarly in deep snow The prius will work harder and it will show in the mileage figures.
