Drive a gas-guzzler in Madrid? You'll have to pay extra to park.

Madrid is using 'smart' parking meters in which drivers enter their license plate number to verify the model and make of the car. If someone drives a diesel or a less fuel efficient car in Madrid, then the driver is charged 20 percent extra to park.

European cities aren't always that easy to navigate when you're in a car. They twist and turn through narrow, busy streets constrained by the topography and architecture, designed long before powered vehicles were prevalent.

If that's not enough, one European city – the Spanish capital Madrid – is charging gas-guzzlers extra to park.

Curb-side parking in the city now uses 'smart' parking meters, according to Bloomberg Businessweek (via The Car Connection).

Once drivers have found a space, they must then enter their license plate number. The meter is connected to a central database which determines the make and model of the car you're driving, and charges drivers accordingly.

If you're lucky enough to be driving a hybrid or other fuel-efficient model – already saving money on gas and taxes, in Europe – you can also reduce your parking bill by up to 20 percent.

If, on the other hand, you're driving a diesel or an older vehicle and pumping out extra pollution as you drive around the city, there's a 20 percent surcharge.

There are even congestion fees in force. If you're parking on a particularly busy street, a further surcharge can be levied.

The city has actually been installing smart meters since 2006 and has amassed 4,000 of the units, but has only just begun to use their full functionality. The new fee system came into force from July 1.

Madrid has some of the poorest-quality city center air in Europe, and it's this that has convinced the city to take action on older and dirtier vehicles.

Just as pollution-based regulations have led drivers in other cities to opt for cleaner vehicles, Madrid is hoping its parking regulations will do the same.

Others aren't so convinced. Mariano González of Ecologists in Action said the move could penalize drivers of older, but still relatively clean cars. A large modern SUV might pay standard parking fees, but a minicar or subcompact from a decade ago, still far more economical, might have to pay extra.

If the system proves successful though, we'd not be surprised to see other European cities adopting similar ideas.