Everything a tyre can tell you about itself is written on its sidewall, in a code nobody explains when you buy a car. Most drivers know the size is in there somewhere. Fewer know that the two characters at the end of it decide what the tyre is allowed to carry and how fast it is built to run.
Here is the whole marking, decoded, using a common South African size as the worked example: 215/65R17 99H.
What do the numbers on a tyre mean?
They are four separate measurements read left to right. 215 is the tyre's section width in millimetres. 65 is the aspect ratio, meaning the sidewall stands 65 percent as tall as the tyre is wide. R stands for radial construction, which is what almost every passenger tyre is. 17 is the rim diameter in inches.
Taken one at a time:
215, the section width. Measured in millimetres across the widest part of the inflated tyre. A bigger number is a wider footprint.
65, the aspect ratio. A percentage, not a measurement. The sidewall height is 65 percent of 215 mm. Drop that number and you get a lower-profile tyre with a shorter, stiffer sidewall, which is why low-profile tyres transmit more of the road into the cabin.
R, the construction. Radial. You will occasionally see other letters on older or specialised tyres, and sometimes ZR on high-performance sizes.
17, the rim diameter. In inches, awkwardly, on a marking that starts in millimetres. This is the number fitment centres ask for first, and the one our own pricing calculator uses.
That is the size. The two characters after it are a different kind of information.
What is a tyre load index?
The load index is the number immediately after the size, and it is a coded reference to a maximum weight per tyre held in tables published by tyre manufacturers. In those tables, index 99 corresponds to 775 kilograms. The tyre does not print the kilograms on the sidewall. You look the index up.
This is the marking that matters most and gets checked least. Four tyres at index 99 are rated to carry considerably more than four at index 91, and the difference is not academic once a vehicle is loaded.
Three cases where it earns attention in South Africa:
- Bakkies. A load bin invites use. The tyres fitted at the factory were chosen for the vehicle's maximum mass, and a cheaper set in the same size with a lower index quietly reduces what the vehicle can carry.
- Towing. A caravan or trailer puts extra weight through the tow vehicle's rear axle, on top of whatever is already in the boot or bin.
- Electric vehicles. Battery packs are heavy, and EVs are often noticeably heavier than the petrol car they replaced. Tyres fitted to them frequently carry higher load indices and reinforced construction for exactly that reason.
If you are replacing tyres, match or exceed the index on the placard rather than the index on whatever is currently fitted. Someone may already have fitted the wrong thing.
What does the speed rating on a tyre mean?
The letter at the end of the marking is the highest sustained speed the tyre's construction is rated for while carrying its rated load, read from a published table. H corresponds to 210 km/h and V to 240 km/h. It describes what the tyre is built to withstand, not a target.
The rating exists because heat is what destroys tyres at speed. Fitting a lower rating than the vehicle was specified for changes the tyre's rated capability, and it is worth knowing you have done it rather than discovering it later. Fitting a higher one is generally harmless and sometimes stiffens the ride slightly.
The other stamps on the sidewall
The DOT code. Ends in four digits: week, then year of manufacture. 3219 is week 32 of 2019. Rubber ages whether or not the tyre turns, which is why this matters most on a spare or a caravan tyre. The full method is in the spare tyre problem.
M+S, or a three-peak mountain symbol. Mud and snow, or severe snow service. On South African roads the M+S marking usually turns up on all-terrain tyres fitted to bakkies and 4x4s.
XL or Reinforced. A tyre built to carry more load at a higher inflation pressure than the standard version of the same size.
Tread wear indicators. Small raised bars sitting in the grooves at intervals around the tyre. Look for the little triangles or the letters TWI on the shoulder and they point to where the bars are.
Maximum pressure. Moulded somewhere on the sidewall, and this is the one that causes trouble. It is the tyre's maximum, not your vehicle's recommendation. Your numbers live on the placard in the driver's door jamb or in the owner's manual, and we set out why the difference matters in tyre pressure: the two-minute check.
One photograph, and we can quote you
Our treatment is priced by rim diameter, which means the number after the R is the only thing we need to give you a figure. Photograph the sidewall so the full marking is readable, send it to +27 76 089 4066 on WhatsApp, and you will get the price for your size back the same day.
While you have the phone out, take one of the tread as well. Between the two pictures we can usually tell you what we would treat, what we would not, and whether there is anything on that sidewall you should be looking at before you spend money on anything. If the set is sound and you want the morning, the booking page has the calendar on it.