235/40 R18 Tyres Explained

235/40 R18 is a common tyre size for performance orientated saloons, sporty hatchbacks and larger family cars. The numbers are simple. 235 is the tyre width in millimetres. 40 is the aspect ratio, so the sidewall height is 40% of 235mm (about 94mm). R18 means a radial tyre that fits an 18 inch wheel. You'll find this size from premium manufacturers like Michelin and Continental - for example the Michelin Pilot Sport 5 and Continental SportContact 7 are available in 235/40 R18. mid range choices include Hankook, Kumho and Falken. Value options on the page include Westlake, Tomket, Mazzini and Wanli. To confirm this is the right size for your car, check the tyre sidewall, the driver's door sticker or your vehicle handbook. You can also enter your registration on Blackcircles to see exact matches and recommended load/speed ratings.

The tyre size that you find on the sidewall of a tyre will tell you important information such as the width, profile and diameter of your tyre. In this case, for the tyre size 235/40 R18, this means the following: 235 - the width of the tyre in millimetres. I.e. 235mm wide. 40 - the height of the sidewall as a percentage of the width. In this case 40% R18 - the diameter of the tyre's inner rim in inches. Basically what size of wheel this is designed to fit onto. In this instance it is a 18 inch wheel.

There is no standard recommended tyre pressure for 235/40 R18 tyres, as each vehicle manufacturer will recommend their own optimal tyre pressure so you can enjoy the best performance from your vehicle. If you are unsure as to the correct tyre pressure for your 235/40 R18 tyres, there are a few different places you can look to find this information. Vehicle handbook - There is a section in most manufacturer handbooks which covers all the technical details for a tyre. This includes the tyre size, load rating, speed rating and tyre pressure. Tyre label - The tyre label is often found on the door jamb of the driver’s side. It is a small rectangular sticker which contains the information you need to know in regards to tyre pressure and tyre sizes. It should also tell you the tyre pressure recommendation

The height of your tyre's sidewall for the tyre size 235/40 R18 can be worked out by using the tyre's profile size. For the tyre size, the profile is 40. What this tells us is that the sidewall height is 40% of the cross-section width of the tyre. The sidewall height is determined by the distance between the rim line and the tread line. So, as this tyre has a cross-section width of 235mm, the height of the sidewall is 40% of 235 – which is 94mm.