I'm attaching a link to this awesome site:
Browse All Smart Fortwos | Fuelly
This is a great site for people who want to keep track of their mpg's. At least one of our members other than myself is already using it.
It's free, and the link above shows the (currently) 58 smarties already listed there. Mine rules, both in # of fuel-ups (92) and mileage (nearly 45 mpg). I think that anyone obsessed with mileage enough to record their data on this site will agree that it's pretty much impossible to get over 48 mpg consistently and that those calculations that are based on just one tankful are probably not very reliable.
BTW, a graph of my fillups shows that my 'worst' mileage (still very good) came in the cold months. I'm guessing that this was caused by the gas mixture (tho I usually get my gas in 10%-ethanol-using New Jersey) and the worse performance of the cold engine. The difference was small but prolly not due to chance (dipping from 45 to about 43).
On my current tank, I have 0.9 gal to go and am already at about 355 miles. Hooray for warm weather! (And it helps that I'm now driving even slower than usual.)