According to Parker a 3 door ST3 weighs 1317kg and has a fuel tank which holds 55 litres
Density of petrol ranges from 0.71 - 0.77 so it weighs 710 - 770kg per cubic metre. Take the midpoint at 740kg per cubic metre and it works out that half a tank of petrol (22.5litres) weighs 16.65kg
I personally weigh 74kg so all together my car with me in it and half a tank of fuel weighs 1407.65kg
Geek I know
Chris