Fuel efficiency in continental Europe is measured in liters consumed per 100 km, which is the inverse of the UK/US standard miles per gallon. To convert from liters/100 km to miles/gal, divide 235 by the value. The same equation can be used to calculate miles/gal to liters/100 km as well. For example, if your European car gets 6.2 liters/100 km, that's 235/6.2 = 38 mpg. Another example: 22 mpg = 235/22 = 10.7 liters/100 km.
The table below gives you conversion from liters per 100 kilometers to UK and US miles per gallon. You'll notice that for l/100km the key is to have as small a number as possible.