Why Overfilling the Tank Is a Bad Idea

Most drivers fill right to the top to save a trip later. Filling to the brim carries real risk, though, and here is why.

Manufacturers leave headspace in the tank for safety and to allow for thermal expansion, so the "fuel tank capacity" in the handbook is a safe figure, not the physical volume. Fill right up and you may add more than the stated capacity, which is why some drivers rock the car to squeeze in a round number. That is not a good idea.

1. It damages the charcoal canister

Overfilling lets liquid fuel reach the lines of the evaporative emission system. The carbon in the canister has far finer pores than ordinary carbon, so liquid petrol cannot escape once it enters: the carbon is saturated and the canister loses capacity. Surplus fuel can also travel down the lines into the intake, fouling the spark plugs, causing stalling or preventing the engine from starting, or spilling out of the canister and wasting fuel while polluting the environment. The canister exists to capture fuel vapour, and repeated overfilling can write it off early.

2. It is a fire risk

The tank has a vent to prevent it splitting as fuel expands. Overfill and fuel easily escapes through that vent. Wasting fuel is the least of it: if the spill meets a spark or flame, it can cause a fire.