If you're in the UK, there are a couple things you need to do. First, when you set up your car, make sure you choose Litres as your reporting units. Here's what that option looks like:
You want to choose Litres instead of the default US gallons.
Then, make sure you click the "UK" link in the upper left corner of any page. (more info
.) Once you click that, the site will show you UK MPG instead of US MPG.
You might also take a look at our FAQ: I'm in the UK. Will Fuelly work for me?
At the bottom there's a link to an offsite FAQ that has even more tips/tricks for Fuelly users in the UK.