Free Range per Tank calculator with clear step-by-step results.
Theoretical range assumes you run the tank dry, which nobody does. Deducting the reserve you will not touch gives practical range, and the load penalty accounts for the fact that a fully loaded or towing vehicle can consume 10-30% more than its quoted figure.
Effective consumption
L/100km = quoted consumption x (1 + load penalty)
Practical range
km = (tank capacity - reserve) / effective consumption x 100
Enough to reach the next station comfortably - often 10-15% of tank capacity. Running very low also risks drawing sediment and, on some vehicles, overheating an in-tank fuel pump.
Aerodynamic drag rises with frontal area and rolling resistance with mass. A caravan can easily add 30-50% to consumption, far more than the load penalty needed for a full car.