Can I put tyres on my car that are smaller than the original ones that came with the car?

Question from visitor, 08/01/2010 :Can I put tyres on my car that are smaller than the original ones that came with the car? What are the risks?
Answer by rezulteo, 08/01/2010 :

Every car comes with tyres optimized for the size of the wheel, rims, performance, braking system and weight of the vehicle. By having a different size tyre fitted to your car you risk your car's safety. There are also legal requirements concerning your tyres. According to the law, you must respect the size, load and speed rating recommended by the car manufacturer. If not, you risk failing the MOT test.If it is a winter tyre, you can lower the speed index and go from V to H.