I've been having some bad gas mileage on my 2005 Prius and for the longest time, I haven't been able to figure the problem. One of my 2005 cars would get 52mpg while the problematic one only gets 40mpg on a good day. Both are maintained meticulously and everything is up to date. Only difference, the 40mpg car has 17" wheels. But that should not reduce the mpg by 12 miles. I went and checked the spark plugs on the problem car and realized they had new Denso plugs in them, ones I bought from the dealership when I got the car a year ago. Just for fun, I decided to put in some old NGK spark plugs to see if things would change. Drove the car a couple days now and I'm up to 47mpg with the OLD PLUGS. Same driving pattern, nothing has changed. I'm not sure why the Toyota Denso plugs would do this, I have no way to explain it. But now at least my cars are back to normal.