It is dangerous to fill petrol tanks to the brim and consumers shouldn't even top off their gas tanks to the nearest dollar, advised the US Environmental Protection Agency (EPA).

On the US government agency's website, it said extra room in the fuel tank is needed to allow the fuel to expand.

It noted: 'If you top off your tank, the extra gas may evaporate into your vehicle's vapour collection system. That system may be fouled and will not work properly, causing your vehicle to run poorly and have high gas emissions.'

It also highlighted that gasoline vapours are harmful to breathe as they are a source of toxic air pollutants, which are also harmful to the ozone.

'Evaporation from the spillage of gas from overfilling can occur, contributing to the air pollution problem. Remember that you pay for the gas that evaporates or is spilled on the ground,' the statement noted.

And you're likely to spill when you try and overfill your petrol tank.

Petrol company ExxonMobil said it does not endorse such practices.

Its spokesman said: 'The auto-stop mechanism is put in place for the convenience of the user ... (who) would not need to hold on to the pump during the filling of petrol. The mechanism also helps to prevent spillage from overfilling.'

Similarly, fuel specialist Philip Chee of Chevron International said it's important to have some air space in a petrol tank.

If there isn't air allowance in the tank, the carbon canisters inside could get wet and become ineffective.

'If the canister becomes inoperative, the car may emit obnoxious gas and pollute the environment,' he said.

A spokesman for the Automobile Association of Singapore said of the cut-off device on petrol pumps: 'As a safety measure, all filling nozzles at petrol stations have a built-in cut-off device that is intended to reduce the possibility of spillage during refuelling.'

'We do not encourage motorists to maximise savings on petrol by refuelling with their cars tilted. Not only does the overflow of petrol increase fire hazards, petrol spillage is also detrimental to the paintwork.'

Pay only for what leaves nozzle

You may have seen the e-mail: don't fill your tank to the brim because the excess petrol flows back into the pump.

The warning is that you end up paying for it.

Online forums have been rife with discussions about petrol station benefiting from such excess fuel after the automatic trigger is released whenthe tank is full.

Some netizens have even posted graphics online to show how this is supposedly done.

So The New Paper on Sunday checked with the experts and it turned out to be an oily urban legend, at least for Singapore.

The mechanism that is installed in these petrol pumps is not found here, but in some other countries such as the United States.

There, some petrol stations have pumps equipped with a return valve to prevent harmful gas vapours being released into the environment.

In Singapore, the payment counter jumps only when fuel leaves the pump nozzle, and not while it is in the hose. As the sensor at the nozzle measures the amount of fuel pumped out, consumers only pay for that 'out' amount, not the 'in-hose' amount.

NO RETURN VALVE

There isn't a return valve in the pump hose, as suggested by some netizens. Singapore does not have this system yet.

Petrol companies Exxon Mobil, Shell and Chevron all said petrol does not flow back into the station's supply storage.

Spring Singapore (Standards, Productivity and Innovation Board), which checks on pump accuracy, confirmed this to be true.

A spokesman for Spring explained: 'A petrol pump consists of a nozzle and hose. When the tank is full, the sensor on the nozzle will automatically cut off the supply. There is no backflow. Consumers only pay for the amount dispensed from the nozzle.'

The spokesman said the board conducts these tests regularly.

She said these tests involve pumping oil from the petrol pump into a standard tank measure (25-litre capacity), and checking that amount against the petrol dispenser reading.

The process is repeated three times to verify that the reading corresponds to the amount dispensed by the pump.