Frequently Asked Questions About Tyres
What does 235/40 R18 mean?
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.
Yes, tyre sizes need to be exact. Tyres are an integral part of a vehicle and so require that they are produced to exact standards. As well as being the only part of a vehicle that is in contact with the road, many components of a vehicle rely on tyres.
The speedometer requires the tyres to be precise so that it is able to accurately calculate the speed at which you are travelling. If the tyres are not exact or the wrong size you will get incorrect readings.
Also, the handling, braking and suspension components of a vehicle requires accurate tyres so that they can deliver an optimal performance. Should the tyre size be inexact, your vehicle would not perform well and you could also find your safety when driving is compromised.
In a lot of cases, especially for front-wheel drive vehicles, the recommended tyre size from the vehicle manufacturer will be the front and back tyre sizes are the same.
However, this is not always the case. Many rear-wheel drive vehicles often have the requirement for wider tyres on the read axle, this is to help with greater traction and optimised performance.
To find out what the manufacturer of your vehicle recommends, the best thing to do is check your vehicle handbook. This should have all the information you need in regard to which tyre sizes are required for your vehicle.
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