Plans and techniques used included cooling the fuel down as much as possible by leaving the jerry cans covered in cloth on the grass next to the swimming pool at the resort where we stayed in Messina.
We then soaked the grass and cloth with water and let nature do its course with temperatures near freezing point in the early morning.
Just like an alcohol thermometer, petrol and diesel contracts or expands with temperature change.
Our aim was to contract it as much as possible before filling the fuel tank.
Tyre pressures are set at the absolute maximum recommended level to minimise the rolling resistance of the car.
Another trick used was parking the car on a slope with the filler cap at the highest point to get as much fuel as possible into the tank.
http://southcoastherald.co.za/156149/zimbabwe-to-cape-town-and-beyond-on-a-tank/