Self-service gas stations provide the onus of filling your gas tank up to you rather than have an attendant take care of it for you. When you're filling your gas tank by yourself, the nozzle usually clicks off in an automatic fashion.
This indicates that your tank is already full and you shouldn't add any more gas to it or else it might overfill and present an environmental or fire hazard. These standard-issue pumps turn off when your tank is at a certain level for your own good.
However, some motorists don't want that. When they say fill 'er up, they mean it. However, the level is there to leave room fro gas expansion. If you don't have space for that, then you're likelier to waste fuel and produce harmful fumes in the environment.
Is Topping Off Bad for The Environment?
You Need Space: Like in a human relationship, your gas tank's fuel needs space. This extra empty space also enables your vehicle's emissions system to work properly. This enables more efficient usage of fuel as well. The system is made to re-burn and collect fuel vapors to reduce the emissions from your car. It makes it more efficient.
Protecting the Charcoal Canister: When you top off your tank so much that there's a reduced air dome, raw gasoline can end up the automobile's evaporative emissions system, specifically an important part of it known as the charcoal canister or carbon filter. This is only designed for gas vapor carrying. It doesn't deal with liquid gasoline so good. It can malfunction with that overfull gasoline level and whatnot.
Don't Allow the Evaporative Emission System to Run Afoul: Raw fuel can make our evaporative emission system not work well. So don't fill 'er up too much. What's more, an overflowing tank can end up spilling the gasoline into the ground or cause issues with your motor operation, especially when the fuel ends up entering your canister or filter. This causes poor emissions and fuel mileage when all is said and done.
More About the Dangers of Emissions
Carbon emissions are dangerous and states like California are already doing what they could in order to reduce emissions from vehicles. It has also gotten to the point where car designers have intentionally made it so that bad habits in filling up your tank can lead to vehicle malfunction. This should discourage you from polluting the environment just because of a mistaken belief that you're getting more bang from your buck by filling your tank to the fullest.