1. The speed rating for a tyre fitted to a vehicle must be at least as high as the maximum legal speed applicable to that vehicle. So for cars for example, tyres must be rated to at least 100 kph. this really is not a problem as virtually all car tyres are speed rated well in excess of 100 kph.
2. Tyres on the same axle must be the same size, construction and tread type. i.e both radial or both cross ply, both 4 ply or both 8 ply etc, both directional or both asymmetric or both symmetric.
3. The minimum legal tread depth limit is 1.5 mm for car and van tyres. However, many agencies change their tyres at 3mm and we would recommend replacing your tyres at or before 2mm. (note: Tyre performance in wet weather deteriorates when tread depth is below 4 mm). The tread must extend at least three quarters of the width of the tread and around the entire circumference of the tyre
4. Load ratings must be within 2 numbers of the tyre on the other side of any given axle. e.g. if the existing tyre has a load rating of 82, you can fit a tyre of any load rating ranging from 80 to 84 to the other side of the axle. Wof regulations permit you to have, for example, 2 directional tyres at the front and 2 asymmetric tyres at the rear but 2 different tyre pattern tyres must never be on the same axle.
5. All puncture repairs comply with regulations with no illegal string repairs, no sidewall repairs and no repairs too close to the sidewall.
6. Snow and Winter tyres can only be fitted as a set of 4 and that the tread depth must be 4 mm or higher. They can not be mixed with standard road tyres.
Usually it is a good idea to buy two tyres at a time and of course 4 new tyres is also always the best way to go. Buying 4 tyres ensures you get a matching set and makes sure you can not get faced with an out of stock situation when you go to buy the other two.