one cubic metre of water weighs a ton. that's what a ton is. i know petrol is lighter than water, dunno about diesel.
To meet EN590, which is the European standard for diesel, the density must be between 820-845. So, 45l would weigh 36.9-38.025kg.
I keep my tank fairly empty because it feels loads lighter that way. Now I know that a full tank weighs less than my dog, I'm not so sure anymore. []