What happens when use wrong tire size?

I'm English so I say TYRE. Tyres have three size measurements. Lets say 185x50x15 for example. 185 is the width of the tyre measured in cm. 50 would be the 'profile' which is the distance from the inner diameter to the outer diameter also measured in cm. So anything less than 50 is a low profile tyre. The 15 is the diameter of the wheel in inches. If you alter the outer diameter of the profile of the tyre as used in this example, your speedometer will read incorrectly as that is governed by cogs inside the gearbox. A 10% difference is acceptable here in the UK in order to pass the MOT test, but if the new tyres alter the speedometer too much you might have to change the ratio of the cogs in the gearbox in order for the car to pass it's MOT. Car manufacturers go to great lengths to make cars safe and economical, minor changes may be acceptable, but if you go extreme, please be careful cornering and running the vehicle to the max. Hot, Cold, wet, or fully loaded and you could be in trouble.