I would like to add that if you are flying a cross country then if you do not lean in line with the POH the fuel rate may be much higher than the POH figures, and it may substanially affect your fuel calcs (this may be an obvious comment to many on PPrune but perhaps not a low hour pilot)
I was reminded this by a very experienced CAAFU friend who had been asked to be an expert witness at an inquest into the death of a pilot whose areoplane had run out of fuel much earlier than expected and the only explanation that they could find was that the aircraft wasnt leaned in line with the POH which increased the fuel burn - he didnt make his destination and died in the forced landing attempt
It did make me think....