Determining the fuel consumption is always on the todo list of the teams. In the past, test day has always been a good indication of the number of laps that cars can do with a full tank.
For instance, last year Paul Truswell observed from the test day that Audi could only do 11 lap stints whereas Peugeot could do 12 lap stints.
source:
http://trussers.blogspot.com/2011/04...-times-at.html