I purchased my 07 Prius a year ago with 60k miles on it. I do a lot of driving and when i first purchased it i would have the cruise control set at 80 and i would constantly get 55-60 mpg. 6 months ago it dropped to an average of 41mpg so i replaced the spark plugs with the same plugs that were in it and replaced the air filter. This did not help at all. I have 93k miles on the car now and my last to tanks have only averaged 31-35mpg does anyone know what has caused this and how to fix it? My driving habits have not changed the tires are inflated at the specs on the door and I always have the AC on high but even with the ac off it doesn't make a big difference. Thanks in advance.