On a mud tyre, the tread is much deeper than standard tyres. The tread on these tyres is most commonly seen in chunked or blocked patters, to help the tyres withstand and hold more mud. They are designed to dig into these difficult surfaces, and clear more quickly, allowing for more mud to go in and then out. As a type of vehicle tyre they are most often seen on four-wheel drive trucks or specialized vehicles. Though commercial vehicles are not usually sold with mud tyres, they can be added as an option aftermarket, or customized. They are common on racing and off-roading vehicles, and come specially equipped on most military vehicles. Mud tyres are often too large for standard sedan-sized cars, and so are rarely seen on them, unless the car has been altered or lifted significantly. Mud tyres on many vehicles can be seen with mud flaps, which are tough rubber panels that hang from the car beside the tyres. Because of the mud moved in and out of the tyres, whether they are stuck and spinning or in regular motion, the areas of the car surrounding the tyre tend to get very muddy. The mud flap helps to absorb the mud from the tyres, and keeps the rest of the car from getting clogged, including the inside of the wheel wells.