I don't but I do tank fuel to full every time when 'on road' through Europe with my little ForFour and it is cheap. I can do about 1100km on one full tank. Which is 4.2 - 4.3L / 100km (66-67mpg). When car is fully loaded, but nothing on the roof. Through lots of hilly highways in west Germany, not only flat... Awesome.
Love that engine. 1.5 CDi