The size of most tyres is marked on the sidewall with a whole lot of numbers and letters. Most of them don’t seem to tally with either centimetres or inches. Why so mysterious?“Mystified”.
The markings on most tyres these days are internationally similar, but they still carry vestiges of more than 100 years of muddled imperial/metric history and detailed differences, additions and omissions between different markets and makes.
There are several dozen parameters, and even the briefest decoded listing of definitions would run to ten pages of small print! If you want to know and understand it all, there is a recommended reading list of…40 books!
The sidewalls of most tyres each have up to several hundred letters and numbers as words or codes. These include multi-dimensional measurements of size, casing materials, multiple ratings for speed, pressure, load and temperature, manufacturing dates, warnings, brandings, tread design priorities, vehicle suitability etc.
Overall they give the tyre’s “specifications” in a mixture of clear statements and coded riddles – the diameter of the wheel it fits (in inches), its tread width (in mm or tenths of an inch), its aspect ratio (tyre wall height as a percentage of width), its construction type (radial or cross-ply/diagonal), its tread and sidewall ply numbers and materials, its tread pattern, its load and speed and ratings etc.
Some of this information is coded; sometimes it is clear. Either way, most of this is not essential to the average motorist, who will either read none of it or only the primary indicator - something like 205/65R15. Sometimes with an additional S or H or V before the R to denote maximum sustainable speed.
The 205 is the width of the tread, the 65 is the “profile” (or aspect ratio), the R means it is a radial tyre (a crossply will have a D standing for diagonal ply) and the 15 is the diameter of the wheel rim it must be fitted to.
The rim itself will probably have that number etched or embossed, and a “J “number (eh?) specifying its width, which must be compatible with the tread width.
Generally, you can fit a tyre one step wider or narrower than the precise J, but any tyre outside that limit will require a change of rim.
The speed rating indicates the maximum speed the tyre can safely sustain for extended periods.
If the additional marking is an S (or nothing) the tyre will have a standard speed rating, which is well over 160 kph. H and V are necessary for cars likely to be driven at more than 200 kph and up to 250!
Max pressure is usually marked as such, with either kilos or pounds indicated. So are max loads (per tyre), athough some markets use a two-digit numeric code. Be aware that is the “maximum” pressure for the tyre under any circumstances, not the recommended pressure for regular use.
Multi-dimensional measurements of size, carcass materials, multiple speed and pressure ratings are included in the letters and numbers on the tyre walls.
Photo credit: Shutterstock
The DOT number is the date of manufacture, usually four digits representing the month and year the tyre was made. Today would be 0924. In some markets this is crucial, because tyres more than 10 years old are banned, and inspectors will get sniffy about any tyre more than six years old.
The ply ratings are usually in plainspeak, stating how many there are in the tread and how many in the more flexible sidewall, and whether they are made of polyester or nylon or steel or a mixture of all three. Those that contain a layer of Kevlar (!) will be sure to say so.
A total of 6-ply is relatively light and comfortable, but not particularly robust; 12-ply is heavier, more expensive and much more resistant to thorns and sidewall cuts... and your backside.
A few decades ago, aspect ratios were routinely 83. Modern “low profile” tyres are more frequently 75 (on big 4x4s) and 65 (on cars) and can go as low as 25 (even on Chelsea Tractors!).
There is also a niche market for especially high profile tyres going above 100 (specialised off-roaders). The effect of this range on road clearance and tyre circumference (for riding potholes and steps) is not insignificant.
Lower profiles are lighter and improve stability and grip but have a harsher ride because their sidewalls flex less and the cushion of squidgy air is thinner.
Other letterings, usually acronymic, include M+S (for mud and snow) AT (for all-terrain) etc. These are references to the tread pattern and depth.
In semi-polar markets there are distinctions between Summer and Winter tyres, because tyres that operate well below freezing need a different rubber compound.
The rubber compound used is rarely indicated; harder compounds give less grip but last longer. Retreads usually have softer compounds (and wear very rapidly) because they cannot be “cooked” like a tyre in original manufacture.
Almost all car tyres (big and small) are now tubeless. Adding a tube is not necessarily helpful nor recommended (the inside surface of tubed tyre casings is different to avoid chaffing), but inserting a tube is one way of repairing a puncture in an emergency. Tubeless tyres cannot be fitted to wheels with split rims (older 4x4s and most trucks).
