Tyre Laws & Information
Tread Wear Indicators
All tyre manufacturers build a tread wear indicator into the tread pattern. This a small rubber moulding raised 1.6mm above the base of the tread groove so that when the adjacent tread has worn down to this level it indicates the tyre should be changed. A few tyres, e.g. some Michelin TRX patterns, have an additional indicator at 3mm to indicate the tread depth is getting near the limit. Most car and light van tyres have 8mm tread when new.
Minimum Tread Depth
The requirements in the UK are that the grooves of the tread pattern of every tyre fitted to a vehicle shall be of a depth of at least 1.6mm throughout a continuous band comprising the central three-quarters of the breadth of tread and round the entire outer circumference of the tyre.
A tyre used on a vehicle or trailer must not have any cut or damage to the exterior of the tyre which is more than 25mm or 10 percent of the section width, which exposes the cords or ply of the tyre or any internal or external lump bulge or tear caused by separation or partial failure of the structure. It is a requirement of using a vehicle on the road that the tyres fitted shall be in good condition and suitable in all respects, including inflation pressure, for the use to which the vehicle is being put.
It is an offence to use a tyre which has any defect (not just illegal tread depth). The penalty for an offence is a fine at level 4 of the standard scale (currently upto £2,500), compulsory 3 point endorsement and discretionary disqualification. Each faulty tyre is a separate offence.
All tyres fitted to modern vehicles are tubeless radials. A radial tyre is inherently more flexible than the original cross ply tyre of 30 or more years ago. It gives better cornering grip because the sidewalls can bend whilst still allowing the flatter tread area to remain in contact with the road.
Tubeless construction gives safer deflation characteristics in the event of a puncture, though a few “tube type” tyres are still available for special purposes. It is never recommended to fit tubes to a tubeless tyre: though sometimes due to severe corrosion of the wheel a tube must be fitted to get it to hold air. (The correct procedure, though, is to clean the wheel, and repaint if necessary, to ensure an airtight seal between tyre and wheel).
Tyres for commercial vehicles have stronger sidewalls to carry a greater weight. As a rule, tyre pressures should be checked once a week. Correct maintenance of tyre pressures and wheel alignment is essential to maximise tyre life (as well as safe and comfortable handling of the car).
The major tyre companies have on-line sites with further information on tyre charastics and selection eg.
We do not recommend remoulds, but see the Retread Manufacturer’s Association web site for further information.
Modern tyre sizes consist of 3 numbers: the width of the tyre, the profile (aspect ratio) and the wheel size: so a tyre size 155.70.13 is 155 mm wide, has a sidewall which is 70% of the width and fits a 13 inch wheel. The tyre size is decided by the vehicle manufacturer. More powerful cars require wider tyres to give more grip (to handle the power output and for better cornering ability). Larger cars also need bigger wheels and tyres to handle the weight of the car.
Tyre profiles range from 85% down to 35%. The lower the profile of the tyre the better the road holding when cornering. This is because the narrower sidewall of the tyre has less room to flex. Low profile tyres tend to give a bumpier ride because the tyre has much smaller sidewalls to cushion minor surface irregularities.
Do not fit a lower profile tyre than originally specified unless you also increase the wheel size to compensate for it: so that the rolling radius of the tyre remains the same! Failure to observe this precaution means the vehicle speedometer will be incorrect; the vehicle will use more fuel; the ride will be uncomfortable and the vehicle may catch the ground over “speed bumps”. Tyres which are too small may also overheat and your insurance could be invalid.
After the 3 numbers of the tyre size there is usually a number and letter combination which indicate the maximum load per tyre at a certain maximum speed. The load and speed are determined by manufactuer’s design and testing based on the tyre size and tread pattern. A coarse pattern will heat up more quickly therefore a lower speed rating will usually apply at the stated load when compared to a normal pattern.
TYRE SPEED SYMBOLS
For MOT requirements in this country, it is not necessary to fit a tyre with a speed rating to match the original vehicle fitment or the maximum speed of the vehicle. Tyres must be suitable for the purpose for which the vehicle is being used. See our Speed Symbol Chart for detailed data.
Insurance companies might say that a vehicle should be fitted with the tyres having the speed rating recommended by the car manufacturer: but even if you fit Z-rated tyres for speeds over 150 mph they will not insure you if you drive in excess of 70 mph in this country!
The vehicle manufacturer has to recommend a tyre with a rating to match the maximum speed of the car because when it leaves the factory he does not know exactly where the car is going or for what purpose it might be used. You might want to use the car in a country where higher speed limits apply than the UK or take it to a race track at the weekend.
The speed rating means the tyre can be run at it’s maximum speed AND at it’s maximum load for an extended period without overheating.
TYRE LOAD INDEX
Most tyres also have a load index number associated with the speed rating letter. This number can be converted via a table to give the maximum load the tyre can safely carry at the rated speed. See our Tyre Load Chart for detailed data.
The tread pattern is required to disperse water from the road so that the tyre can still grip in wet conditions – otherwise the tyre might aquaplane both when driving and braking. Water dispersal is aided by tiny slots in the tread called sipes. The tread patterns are moulded onto the tyre and generally speaking each manufacturer uses a unique pattern. Most tyre makers market a range of patterns each with different characteristics.
All Weather Conditions
High Speed Use
Off Road Use
Minimum Road Noise
Some of the tread characteristics are required to suit the particular vehicle application but most can be chosen by the driver to suit his/her own requirements.
If a directional pattern tread is fitted it will give better wet weather performance when fitted the correct way, but is not illegal or dangerous when fitted the wrong way round; nor is it necessary for all the tyres on the vehicle to be directional.
Asymmetric tyres are marked to show which side of the tyre should be fitted to the inner and outer sides of the vehicle. They are designed for maximum cornering grip and should always be fitted the correct way round.
Tyres for high speed use may have a tread compound which is softer than normal thus giving maximum grip but a shorter service life.
The major tyre manufacturers have on-line catalogues: eg.
Uniform Tyre Quality Grading
Tyres have many markings on them to indicate different features. Different markings are required for the European market, for N. America, and for other markets. Some tyres are labelled for all sales areas, some are only labelled for an individual market. The Uniform Tyre Quality Grading System (UTQGS) is an American tyre information system, sometimes found on tyres sold in the UK.
Each tyre manufacturer performs its own tests in these areas, following U.S. Government prescribed test procedures. Each manufacturer then assigns grades that are branded on the tyre.
Treadwear grades typically range from 60 to over 500, in twenty point increments. It is important to remember that the actual life of any tyre is determined by the road surface quality, driving habits, inflation, wheel alignment and the rotation it experiences.
To receive a treadwear grade, a tyre is tested under controlled conditions on a government prescribed test course which does not necessarily simulate the actual application for which a given tyre is designed to perform. As a result of these test parameters, there is no accurate way to assign miles of wear to treadwear grade points.
Treadwear ratings are determined on a 400 mile government test course covering specified sections of public roads near San Angelo, Texas. A group of not more than four test vehicles travels the course in a convoy so that all tyres experience the same conditions. Tread groove depths of the tyres being tested are measured after each 800 miles. The same procedure is followed for a set of control or course monitoring tyres.
Upon completion of the 7,200 mile test, the rating results of both tests are compared, and the tyres being tested are assigned a treadwear rating by the tyre manufacturer. A tyre with a treadwear grade of 400 might be expected to last twice as long as a tyre that has a grade of 200 if given the same usage and driving style.
Traction grades indicate the measurement of a tyre’s ability to stop a car in straight-ahead motion on a wet test surface pavement. It does not measure straight-ahead acceleration. It’s important to remember that traction rating tests are performed only for straight ahead sliding on concrete and asphalt surfaces that have a specified degree of wetting which simulates most road surfaces in a rainstorm. The ratings that result from these tests may not apply to cornering traction or peak values of straight-ahead braking force like those experienced in non-skid braking tests. Traction grades range from A to C, with A being the highest attainable grade.
Traction ratings are established on government maintained skid pads. Twenty measurements are taken with an industry standard control tyre on an asphalt surface and averaged. The same number of measurements are made on a concrete surface. Corresponding measurements are then made on the tyres being tested. Once the results of the tests are compared, traction ratings based on government prescribed coefficient levels are assigned to the tyres that were tested.
Temperature grades also range from A to C, with A being the highest. Temperature grades represent a properly maintained tyre’s ability to dissipate heat under test conditions. Ratings are determined by running tyres on an indoor road wheel test under specified conditions. Successive 30-minute runs are made in 5 mph increments starting at 75 mph and continuing until the tyre fails. A tyre is graded C if it meets the minimum performance required by DOT. Grades of B and A represent higher levels of performance than the minimum required by DOT (US Department of Transport).