We just took our first steps toward support for metric units on Fuelly. If you want to enter your fuel-up data in litres and kilometers, you can choose that option when you add or edit a new car.
Car profiles are still very US-centric with miles and mpg featured, but we added a new data point: Average Litres/100 Kilometers. (Here's what it looks like.) This is a standard measure of fuel efficiency outside the US, and we felt it was another important data point that should be available to everyone.
We still don't have a way to add cars from outside the US into our system, but we're working on that now and hope to have it available soon.
Metric support is brand new and there are bound to be bugs. Please post here if you spot problems.
Yep, it's working. You just choose Metric as the option when you add a car. If you have an existing car you can switch it to Metric, but be aware that Fuelly is going to treat all existing fuel-ups at metric if you do that.