There a several key indicators that will help determine when your tyres may need changing.
A noticeable deterioration in performance can often be due to tyre wear. Your car not handling or gripping the road as well as it normally does, or it taking longer to stop when braking can be tyre related.
Because tyre wear, and therefore a decline in performance are gradual, it is recommended to check your tyres fortnightly and ideally have an expert check them at regular intervals.
Legally, it is the driver's responsibility to ensure that the tread on your tyres is not worn beyond the legal minimum limit of 1.6 millimetres over the central 3/4 of the tyre in a continuous band around the entire circumference of the tyre. It is worth noting that The Royal Society for the Prevention of Accidents (RoSPA) recommends changing at 3mm.
There are many different reasons for tyre wear. The illustrations below show the different types of common tyre wear, and an explanation for each.