This has always been a bit of mystery to me. I’d never really paid much attention to all the lettering on the side of my tyres. It wasn’t until I started getting into the whole Porsche scene that I started to pay attention to all the markings on the tires. So I thought I’d share some of information I have learned while considering my tyre options.
This post is about tyre speed ratings. I’ll get into load ratings and other tyre stuff another time.
From what I can work out, tyre speed ratings were originally developed in Europe as a way to assist high performance vehicle owners when choosing replacement tyres to match the speed capabilities of their vehicles.
All tyres are marked with letters to indicate their speed rating. The above chart shows the speed rating letter in CAPITALS and the associated maximum speed.
You will notice that the Z rating is for speeds in excess of 149mph or 240km/h, but there are two different ratings below it. When Z rated tires were introduced, there was a lack of street legal cars that were able to obtain speeds that high or higher. Hence, it was believed that future vehicles would be unable to or would be manufactured so that they could hit those break neck speeds. However, as we know now, there are countless vehicles that can easily go past 149mph or 240 km/h, especially our beloved Porsches. This is why the W and Y ratings were eventually introduced.