2.75" in last 18 hours and will rain off and on for next few days. Driving on a 70MPH Interstate last night at 63 because of age of road and water in lane grooves and got to wondering what it was costing me in MPG to push that water out of the way? And what it was costing those idiots who were blowing by me at 75? I could feel the amount of water they had to expel. My tires are new and grooves are deep. I've had bad things happen when they weren't and the tirerack studies of the worth of tread depth in the rain has more than convinced me to replace early and often. I've felt the difference between all-seasons with longitudinal grooves and rain tires with diagonal water expulsion. Rain tires win big but boy are they noisy. RainX works too. Anyone ever seen any MPG studies specifically on driving in heavy rain?
Simple answer is "it depends". If coupled with a headwind I've lost a good 5 - 7 mpg on a typical drive.
I see it in my C too. Rain/Wet streets can drop my mpg by around 10%. I imagine it's hard to get an exact estimate since rain conditions vary wildly.
I haven't seen a study but in my personal experience, I lose between 5-10 mpg per trip (33 mi). Even with our mild PNW winters, I can average 50-52 mpg (indicated) per commute on dry roads. When it's wet with standing water, I get 42-47 mpg for the same trip.
