How about Rain; will it hurt my mileage?
The first screenshot shows a baseline with no Rain. If you press the * next to 'RRR' you get this screen
If you choose 'Light Rain' in the above right screen, you get the first screen below. Notice the 'AC, Road & Misc Drag' in the black background has increased from 1.49 kW to 4.33 kW (4 lines below the yellow MPG Line). Here we see the MPG has dropped from 47.69 to 40.77 MPG. The below - right screen shows what 'Mild Rain' will do. Really it doesn't even have to be raining; if there is enough moisture on the roadway to cause tires to throw up even a light mist, there will be quite a drop in efficiency. This is because simply elevating the water into the air 3 to 6 feet takes a lot of energy. Water weighs approximately 8.33 lbs per gallon-US. A typically shod Prius with Goodyear Integrity tires will elevate an astounding 1,675,307 lbs of water an hour on a lightly soaked roadway. This 201,117 gallons of water being raised an average of 4.5 feet into the air takes about 2.84 kilowatt hours of energy which agrees with our 1.49 kW usage rising to 4.33 kW & this drops the Prius' mileage from 47.69 to 40.77 MPG. The next screen shows 39.78 MPG & represents the mileage in 'Mild Rain.'
The screen below shows us what a 'Hard Rain' will do to mileage, dropping it from 47.69 to 37.72 MPG. This screen shows that our 'AC, Road & Misc Drag' value has increased from 1.49 kW to 5.91 kW. This is an extra 4.4 kWh being used to get through hard rain soaked roadways every hour. 4.4 kilowatt hours of energy is enough to raise an incredible 2,595,546 lbs (311,590 Gallons) of water 4.5 feet off the highway per hour. The second screen below shows a more finite breakdown of energy usage; notice the 'RRR' (Road Rolling Resistance) value which is 5.07 kW, this is up from 0.65 kW when there was no rain.