keep fuel tank to below half ect.. i always stick about 20-£25 quid in when my thirsty light comes on, takes me to about quarter of a tank.
more fuel you put in, the heavier your car = the more fuel youre going to burn pulling it
I am sure that is wrong, from what I have read. 
Also if you think about it you have to make more trips to the pumps so would you save anything.
to be honest i heared the same too, that the more you fill up in one stop the more you save, but i just can`t see the logic in that, fuel still costs the same to the litre regardless to how much you put in. can anyone break down the theory?..
i know less stops, but still, i wouldnt say that would come into it???
but i`m talking on a power to weight ratio and not fuel economy or laps per tank sort of topic.. drag strip not track if you get me???.
perhaps i should have said, "the more fuel you put in, the heavier your car = the harder your engine is going to work pulling it?..