I just completed a road trip from Maryland to Missouri and back, 875 miles each way, with a 4-cylinder 2007 Camry. Going to Missouri it rained heavily 80% of the way with a strong head wind and temperatures in the 45 to 55 degree range resulting in an average MPG of 28. Coming back to Maryland it was cloudy with only a few showers, a light tail wind and temperatures in the 60 to 75 range; average MPG was 32. Both coming and going my cruising speed ranged between 70 and 75 mph. I have made the above trip more than 50 times in the last 30 years and have seen this phenomenon many times. However, the difference in MPG was less with my previous larger and more powerfull V8 Crown Vic, only 1.5 to 2 MPG. I don't have any plans to make this drive with the Prius, but I suspect the Prius discrepancy would be greater than 5 MPG under the above conditions.
50 MPG = 2Gallons / 100Miles (+ 5%) = 2.1 Gallons / 100 Miles = 47.6 MPG 40 MPG = 2.5 G / 100 M (+5%) = 2.625 / 100 Miles = 38.1 MPG. So, at 50 MPG a Prius loses 2.4 MPG, but only uses .1 more gallons. Another car at 40 MPG loses only 1.9 MPG, but uses .125 more gallons. Similarly, 30 MPG = 3.3 G / 100 M (+5%) = 3.5 / 100M = 28.6 MPG 20 MPG = 5G / 100M (+5%) = 5.3 / 100M = 19.0 MPG 10 MPG = 10G / 100M (+5%) = 10.5 / 100M = 9.5 MPG
Think of it this way. There is water on the road and your tires will pump the water out of their path of travel. They will also pick up water off the road and throw it into the air. All of this requires energy. The energy comes either from your battery, gasoline, or is stolen from your momentum. All of the above will result in lower mpg.
+1 And as your tires wear and your tread depth gets more shallow, the tires can't pump it out of the way and just plow or float on it making your mpg even worse than when new.
