Why You Should Never "Top Off" Your Fuel Tank

The Urge to Round Up

We have all done it. You are standing at the pump, watching the digital numbers roll. The pump clicks off automatically at £49.82. The temptation to squeeze the trigger a few more times to reach a perfectly even £50.00 is almost irresistible.

However, this habit-known as "topping off"-is terrible for both your wallet and your car's delicate emissions systems.

The Charcoal Canister

Modern cars are equipped with an EVAP (Evaporative Emission Control) system. Because petrol evaporates into highly toxic fumes, your car's fuel tank is not vented directly into the atmosphere. Instead, the fumes are routed into a special canister filled with activated charcoal, which traps the vapors until the engine burns them off safely.

Flooding the System

The automatic shut-off click at the petrol pump happens for a vital reason: it leaves exactly enough empty space at the top of your tank for fuel vapors to expand.

If you force more liquid fuel into the neck of the tank after that click, the liquid petrol can overflow down the vapor lines and flood directly into the charcoal EVAP canister. The canister is designed to handle invisible vapor, not raw liquid fuel. Flooding it will destroy the charcoal, trigger your Check Engine Light, and lead to a repair bill of £200 to £500 to replace the system.

The Verdict

When the pump clicks, you are done. Put the nozzle back. If you want to ensure you are getting the absolute maximum value for your £50, don't risk breaking your car-just make sure you found the cheapest possible price on FindPetrol before you started pumping.

Local prices differ far more than most drivers expect. For a worked example, see current petrol prices in Leeds.

Find the cheapest petrol and diesel near you

Fuel prices change street by street, and the station you drive past out of habit is rarely the cheapest. Petrol currently starts at 142.9p a litre against a UK average of 161.4p, across the 7,916 forecourts we track. Search your postcode to see petrol prices near me, or browse fuel prices by area to see what your town is paying.