Johnzimmermann said:
A close friend of mine had her prius was delivered with 17 inch tires. She can not get the mileage over 42 miles per gal.
Same here at first, so I know the feeling.

However, it's not a hopeless/lost cause to keep the 17's. Here's my mpg over my first 10 tanks:
Code:
Prius Calc Diff (Lifetime Avg.)
44.2 38.8 5.4 38.5
44.9 40.6 4.3 39.4
44.1 43.0 1.1 40.6
47.6 47.0 0.6 42.3
47.0 45.0 2.0 42.9
48.3 46.1 2.2 43.5
49.7 55.4 -5.7 44.8
49.2 44.3 4.9 44.8
49.1 48.2 0.9 45.1
49.6 48.2 1.4 45.5
MFD currently shows 53.3 on this tank; had it up to 54.6 before a couple of short trips to the store this past weekend. I expect to have it back up a bit higher after a LONG trip tomorrow. Note: I've also got my tire pressure set at 44/42, and switched to synthetic oil at my first 6 month service about 3-4 weeks ago. Also keep in mind as has been posted elsewhere that very short (5-10 minute) trips will kill your mpg.