Your calculations are based on an error: petrol (if we're talking petrol) is less dense than water. One litre of petrol weighs just over 0.7 kg. Diesel is more dense than petrol, but still less dense than water. One litre of diesel weighs just over 0.8 kg.

This may add fuel (!) to the argument that a full tank doesn't weigh that all much more than a half-full one.

Edited by FP on 06/08/2013 at 23:17

I do about 300 a week and fill up each week, though I did for a month half fill for that reason to find it makes no difference,I still get about the same MPG so I dont worry anymore

I get average 35mpg 2010 honda civic 1.8

Most of us don't want to stop at a filling station more often than we have to. It would take a lot of difference to make it worth the hassle of filling only half the tank. And doing so would be no good for those of us who like to check our car's consumption brim-to-brim.

Frankly I believe this claim that the weight saving is worth going to the gas station twice as often is a load of piffle. I even tried to calculate the maths one time. I don't think I was accurate but it came down on the side of filling by a hefty margin.

I run till the red light is on then fill to brim (I fill on the way out somewhere, so as to use a little up to prevent any expansion spills when the fresh tank warms up).