I've been driving my 2007 Prius for about two months now (2800 miles). I've averaged about 48 MPG so far (according to my paper calculation of gas in vs miles driven). The first night I had the car home, I upped my tire pressure to 42/40 after reading these forums. My first tank was my best mileage at about 52. I failed to check my pressure again until the day before yesterday. My MFD had been consistently reading 48 MPG on my daily commute. After re-upping my pressure to 42/40 two days ago (it had dropped to 40/38), the same commute reads 52 MPG for the last two days! Thank you PriusChat.com!
Check tire pressures whenever the daily average outdoor temp changes more than 10'C/20'F or so, or at least monthly. Tires are ideally checked "cold", which means "not driven more than one mile in the previous hour".
You have not driven the car long enough. I use 36/34 and achieve the same mpgs and variation you report.
<div class='quotetop'>QUOTE(jamarimutt @ Jul 21 2007, 11:04 AM) [snapback]482753[/snapback]</div> The difference between my experienced MPG on different tire pressures is clear. No other variables were changed. Two days, same temp, same route, same speeds, etc. etc.