I've been using
Fuelly for a few months to keep track of my mileage, miles traveled, amount of money spent on gas, etc.
The site does a great job breaking down the information into easy-to-read graphs & charts.
I currently have the highest MPG average of all the registered 350Z owners.
There are no 370s on the site, unfortunately. You guys should join.
My profile is available by clicking the link in my signature.