Unlike water, diesel is not one kilogram per litre. In fact, its only 0.85kg per litre. This is important to know when considering your fuel economy, how large your fuel tank is and whether its worth having a long range tank.
In essence then, 100 litres of diesel will weigh 85kg, 150 litres will weigh 127.5kg and 200L of diesel is 170kg. Of course, you have to add the tank weight on top of this if you want to know the total weight, but diesel is actually lighter than water (and also very critical!).
Calculating your fuel range is hugely important, and knowing that diesel is a little bit lighter than fuel allows you to carry a tiny bit more, to ensure you don’t run out!
Now, whilst this is a minor difference, when you are building a 4WD to tour Australia, every kilogram adds up, and I can tell you from personal experience that fitting a long range fuel tank was hugely worth while, and I don’t regret it one bit.