The name says it all, a tyre that carries on functioning when it’s flat. Most cars that don’t carry a spare tyre such as BMW, Mini Cooper, and Mercedes tend to have run flat tyres fitted on them. So when they pick up a puncture or loose pressure they are able to carry on driving for a while before they get their tyre replaced. In normal circumstances with a standard tyre if you we’re to get a puncture the tyre pressure would go down and it would be near enough impossible for you to carry on driving. A run flat tyre eliminates this problem and allows you to carry on driving without any problems. Most cars will come with a warning sign on their dashboard informing them of tyre pressure loss so you can resume your journey and get your tyre replaced at a later time.
A conventional tyre keeps its shape by the amount of air pressure in the tyre so a loss of air pressure will mean the sidewalls of the tyre will collapse causing the tyre to go out of shape. Run flat tyres have stronger reinforced sidewalls meaning that they don’t rely on air pressure to keep the tyre in shape so if they do loose air pressure the structure of the sidewall allows it to stay strong and in shape.
Apart from the structure of the sidewall there is no other difference between a run flat tyre and standard conventional car tyre, the rubber is exactly the same so you can’t say one will last longer than the other. However it is advised that run flat tyres are not repaired if you pick up a puncture the reason for this is because the sidewalls have suffered constant strain and pressure from running on low pressure so it should be replaced with a new tyre.