Passenger Tyre Markings
South African tyre markings are based on the European Tyre and Rim Technical Organisation (E.T.R.T.O.) markings.
The size markings of a tyre denote the physical dimensions of the tyre and indicate the configuration or shape of the tyre casing.
185/70 R14 82T. The significance of each number and letter is as follows:-
- 185 mm in width
- /70 = Profile - percentage of width. In tyres where the aspect ratio is not shown the profile = 82%.
- R = Radial.
- 14 = Rim diameter in inches.
- 82 = Load Index per tyre at the speed rating.
- T = Speed Rating
Some off-road tyres use the above type size markings while others still use the older American markings, for example - 31x10.5R 15 LT. The significance of these markings are as follows:-
- 31 = The outside diameter of the tyre in inches.
- 10.5 = The section width of the tyre, in inches.
- R = Radial Construction.
- 15 = Rim diameter in inches.
- LT = Light Truck.
A common problem facing off-roaders is interchanging between these two size systems without interfering with the outside diameter where a variation of only 2-3% is acceptable.
A common example would be changing from a 265/75R 15 LT to a 31x 10.5R 15 LT: Determining the outside diameter of the 31x 10.5R 15 LT is easy; convert 31 inches into millimetres. By using the conversion table"inches to millimetres" we find that 31 inches = 787.4 millimetres.
The outside diameter of the 265/75R 15 LT is calculated as follows:
- The section height of the two sidewalls added to the diameter of the rim.
- By using the same "inches to millimetres" conversion for the rim diameter we find that 15 (inches) x 25.4mm = 381mm.
- The sidewall heights are:- 265mm
- Section width x 75% (the aspect ratio) x (two sidewalls):
- 265 x 75% = 198.75
- 198.75mm x 2 = 397.50mm
- Rim Diameter + 381.00mm
- Outside Diameter = 778.50mm
The difference in diameter between the two tyres is 8.9mm or 1.13% which is acceptable.
The trend towards a lower profile has gained momentum in recent years and it is possible to convert from a narrower, higher profile tyre to a wider, low-profile tyre. Always ensure that your rims are wide enough to accommodate the wider tyre. The tyre industry requires a rim width that is 70 per cent of the tyre section width.
Also ensure that the load and speed ratingsof the replacement tyres are equal to or better than the tyres you are replacing. Check that the wider tyre does not interfere with the bodywork of the vehicle.
Low-profile conversions do not necessarily remain within the limitations of the original rim-size. It will be seen that while it is possible to maintain the outside diameter of the tyre, the outside diameter of the rim can be increased by as much as three inches.
A chart of approved low-profile conversions has been compiled by the tyre manufacturers, part of which, showing commonly used sizes in South Africa, is reproduced here.
Load category and speed index
It is legally essential that tyre manufacturers define clearly the capabilities of any given tyre. The letter symbol indicates the maximum speed capability of the tyre, matched to the maximum speed capability of the vehicle - regardless of the speed limit in force at the time, and the numerical code identifies the load carrying capacity of the tyre at the maximum speed.