The code on a tyre sidewall is simple. Here's how to crack it:
Tyre sidewall code: For the main size designation, say, 235/45R17 94Y:
235 on the tyre sidewall code is the maximum section width, not the tread width, in millimetres, with the tyre at its notional operating pressure.
45 on the tyre sidewall code is the tyre’s ‘profile’, ‘aspect ratio’ or ‘series’ – they all mean the same thing. It’s just the tyre sidewall height divided by the tyre section width. It's the ratio of the tyre sidewall to the tyre width, basically. The lower the number, the thinner the tyre sidewall - 35 would be typical for a really high performance tyre. Lower numbers on the tyre sidewall code are stiffer in the sidewall, and better for ultimate handling, but are also more prone to damage the wheels when you hit, say, a pothole.
R on the tyre sidewall code means ‘radial’, which all road tyres are these days. The cross-ply is like the VHS video tape and the dodo – dead.
17 on the tyre sidewall code is the rim (wheel) diameter. Despite metrication almost four decades ago it’s still measured in inches, across the bead seats (where the tyre sits on, and seals against, the wheel).
94 on the tyre sidewall code is the load index rating. You must always, by law, fit tyres that meet or exceed the load index on the car’s tyre placard. That applies to ‘plus 1’ and ‘plus two’ upgrades as well.
Y on the tyre sidewall code is the speed rating. The common tyre sidewall code values are H (210 km/h), V (240 km/h), W (270 km/h), Y and Z (both 300 km/h).
See our post on cracking the code to tell how old your tyres are