To ensure that you vehicle is both safe and fuel efficient you should check the tyre pressure regularly to ensure that all your tyres are set to the manufacturers recommended pressure levels. A modern car is likely to have tyre pressure sensors installed. A sudden drop in pressure on one tyre may mean that you have a slow puncture. Check your tyre for sharp items such as screws or nails that may have punctured it but partly blocked the leak. If your tyre does have a slow puncture, get it replaced as quickly as possible to avoid a sudden blow out.