'Speed Rating' is the speed which a tyre can withstand constantly without flying apart. It is measured by running a tyre, under loaded conditions, against a large drum.
To meet a particular speed rating, the tyre is run for periods of time at different speeds according to the standard (whether European or American).
Illustrated below are common speed ratings and their 'speed symbol':
||TYRE SPEED RATING
||180 km/h (112 mph)
||190 km/h (118 mph)
||200 km/h (125 mph)
||210 km/h (130 mph)
||240 km/h (149 mph)
||270 km/h (168 mph)
||300 km/h (186 mph)
||Above 240 km/h (149 mph)
This speed rating must be moulded onto tyres' sidewalls to comply with the relevant standard. Below are 2 systems of tyre branding in current use:
New car manufacturers are required to fit tyres that can match the car's top speed for obvious reasons. The same used to apply for replacement tyres, but now this has changed. In Australia, replacement tyres must only meet a minimum of 'S', which is 180 km/h, or 70 km/h above the maximum legal speed in any state. The only exception to this is for off-road applications such as 4WD and rally use where 'Q' (140Kph) is allowed.
It is important to understand that speed rating is not an indication of road handling performance in any way. In fact, most methods of raising a tyre's speed rating can have the reverse effect. However tyres made for vehicles with high speed capabilities tend to also require high performance characteristics as well.
Producing tyres with high performance characteristics and speed ratings is expensive.