FuelFrog was updated last weekend with support for metric units. We now support liters, meters, and can show your efficiency in l/100km (lower is better) which gets used by many more sensible countries for monitoring their fuel usage.
Upon releasing this, we got an email from a user in the UK who informed us that they measure distance in miles, volume in liters, and efficiency in miles per gallon. This threw me for a bit of a loop and I had to make some "special cases" for the UK.
In any case, we're now happy to also offer support for our UK users and hopefully provide them data in the units with which they're most familiar.