We just bought a high mileage used 2009 Prius - just love it. But I am baffled by the mpg's in rain. On the flats of Nebraska, during an extensive rainstorm, my mileage dropped to 36mpg at best (60-70mph depending on intensity). Next day, on dry pavement in rolling hills of Iowa, I averaged nearly 48mpg at 70mph. Happened again the next day in Wisconsin, rain mpg dropped to low-mid 30's, back up by 10 mpg on dry pavement. I knew 75mph will cause a hit, but usually less than 5mpg over 70, but the rain equation is a real mystery. Is this common?
No mystery here. Rain and wet pavement hurt the mileage of all cars. It takes energy to pump water out from under the tires, and it also takes energy to accelerate raindrops hitting the car. It's just basic physics. Tom
Drive behind another car. All the water thrown up in the air by the tyres is work that petrol was spent to perform. Fixed energy costs always "appear" to affect the Prius more than other cars because of the low baseline GPM.
Guess I never thought of it as basic physics, coming from a 1997 GMC Suburban that got its 16 mpg whether rain, snow, shine, mountains, or just about anything else. Never better, rarely worse (around town stop and go only 12-13 mpg). Still, a 20%+ hit seems pretty heavy for rain driving. But how can I complain at more than 2x the mileage even in the rain.
Basic for you, maybe. Would air density and humidity (not just the raindrops) also affect mileage? If the air's thicker, it follows that more energy would be required to move through it.
Do some gallonage calculations as I illustrated at http://priuschat.com/forums/gen-iii...mileage-gets-worse-in-winter.html#post1411488. Here are some gallonage examples, for fuel used over 100 miles, notice the delta in absolute # of gallons: 48 mpg: 2.0833 gallons 36 mpg: 2.7778 gallons --> delta of 0.6944 gallons 16 mpg: 6.25 gallons 15 mpg: 6.6667 gallons -> delta of 0.4167 gallons 13 mpg: 7.6923 gallons -> delta (from 16 mpg) of 1.4423 gallons
Also note that rain frequently comes with wind. A stiff headwind at 70+ MPH will have a noticeable effect on MPG as well. As cwerdna has already pointed out, the difference between 16 MPG and just a bit less means quite a bit more gasoline usage than the difference between 48 MPG and 36.
Yes, but here is the part that will bother you: water vapor is less dense than dry air. In other words, air gets thinner as humidity increases. This is the reason that airplanes climb poorly on humid days. Tom
