I drive a '08 Prius at work - company car - standard everything - and over time I avg 44 mpg. I drive a '09 Prius at home - standard everything - and over time I avg 42 mpg. Why? The cars are standard w/ factory tires. My wife also drives our private vehicle and other people at work drive the company car. Still I have noted a consistent difference in my mileage when I am driving on very similar roads. Why the difference? Do some get better mileage. Does the on-board computer vary from one car to another?