Of all the components that make up a car, the tire is the only one that actually touches the road. Every acceleration, every braking manoeuvre and every change of direction is transferred to the ground through four contact patches of surprisingly small size, barely larger than the palm of a hand. Choosing a tire is therefore not a maintenance detail, but a technical decision that affects safety, fuel consumption, comfort and the dynamic behaviour of the car.
The characteristics of a tire are summed up in a series of markings moulded into the sidewall, which look cryptic at first glance but are in fact standardised codes. Take a marking such as 205/55 R16 91V: the elements it is made of are, in order:
- The nominal section width, in millimetres
- The series, or aspect ratio, as a percentage
- The type of casing construction
- The rim seat diameter, in inches
- The load index
- The speed rating
The size: width, series and diameter
The first number, 205, indicates the nominal section width expressed in millimetres, that is the distance between the two sidewalls. All else being equal, a wider section offers a larger contact area and therefore allows more lateral force to be transmitted, but it also brings higher rolling resistance and greater unsprung weight.
The second number, 55, is the so-called series, or aspect ratio, and is not an absolute measurement but a percentage: it indicates that the sidewall height is equal to 55% of the section width. The sidewall is to all intents and purposes an elastic element, working in series with the suspension: a high profile filters bumps better and protects the rim, while a low profile, typical of sports cars, reduces deformation under lateral load and gives a more immediate steering response, at the expense of comfort.
The letter R, in this position, has nothing to do with speed: it indicates the radial construction of the casing, that is the arrangement of the load-bearing plies perpendicular to the direction of travel. Universally adopted on road cars today, it ensures a more stable contact patch and lower rolling resistance than the old cross-ply construction. The number that follows, 16, expresses the rim seat diameter, that is the inner diameter of the tire, given by historical convention in inches: it must match the diameter of the wheel the tire is fitted on.
Load index and speed rating
The marking ends with two elements that describe the operating limits of the tire. The number 91 is the load index: a conventional code, not a value in kilograms, which a standardised table matches to the maximum load a single, correctly inflated tire can carry. Index 84 corresponds to 500 kg, 87 to 545 kg, 91 to 615 kg and 95 to 690 kg: an increase of just a few units therefore translates into tens of kilograms of difference.
The final letter is the speed rating and indicates the maximum speed at which the tire is able to carry the load corresponding to its own index. Here too the correspondence is tabulated:
- Q: 160 km/h
- R: 170 km/h
- S: 180 km/h
- T: 190 km/h
- H: 210 km/h
- V: 240 km/h
- W: 270 km/h
- Y: 300 km/h
Note that the letter H, for historical reasons, does not follow alphabetical order and sits between U and V. Other markings may appear alongside the size: XL or Reinforced indicate a reinforced construction, with a load capacity higher than the standard for the same size; the DOT marking contains, in its last four digits, the date of manufacture as week and year (a code ending in 2324 identifies the twenty-third week of 2024); the TWI marking, on the shoulder, shows the position of the wear indicators moulded into the main tread grooves.
The three families: summer, winter and all-season
Beyond size, passenger car tires fall into three broad families, which differ in the composition of the compound and in the tread pattern.
Summer tires. The compound is formulated to retain its optimal elasticity above a threshold conventionally set at around 7 °C, while the tread features large blocks and grooves sized above all to evacuate water. The result is the best possible grip in mild conditions and low rolling resistance, but below that threshold the compound tends to harden, appreciably reducing grip.
Winter tires. The substantial difference lies not so much in the pattern as in the chemistry: the compound, rich in silica, contains polymers that stay soft even at low temperatures, retaining the ability to conform to the irregularities of the asphalt. The tread is also cut with a dense network of sipes, which multiply the biting edges on snow and ice. Two markings need to be told apart: M+S (Mud and Snow) is a declaration by the manufacturer and is not subject to a standardised test; the 3PMSF symbol, the three-peak mountain with a snowflake, certifies that the tire has passed an objective snow traction test.
All-season tires. These are an acknowledged compromise, with a compound and a tread pattern midway between the two previous solutions. They do not reach the level of a summer tire on warm dry roads, nor that of a winter tire on snow, but they avoid the twice-yearly changeover: a reasonable choice for mainly urban use, in areas with mild winters and limited annual mileage.
How to choose
The first constraint is not technical but documentary: the sizes approved for a given car are those listed on its registration document, and they reflect the homologations carried out by the manufacturer. Fitting a size that is not provided for means altering the relationship between wheel revolutions and actual speed, with consequences for the calibration of the speedometer and for the work of systems such as ABS and ESP.
The second criterion concerns the operating limits: load index and speed rating may be equal to or higher than those prescribed, never lower. The only recognised exception concerns winter tires, for which a speed rating lower than the one shown on the registration document is allowed, provided it is not lower than Q and that a dedicated sticker, clearly visible to the driver, is applied inside the cabin as a reminder of the new maximum speed. It is, however, a derogation limited to the winter period: once that window has closed, refitting tires that comply with the registration document is not optional.
As for seasonality, in Italy the obligation to run winter tires or to carry snow chains on board applies, on the stretches of road covered by the orders issued by the road authorities, from 15 November to 15 April, with a one-month tolerance at each end that allows fitting from 15 October and replacement by 15 May. Beyond the legal obligation, the technical criterion remains temperature: where winter averages fall consistently below 7 °C, a winter tire is worthwhile even in the absence of snow.
A further tool for comparison is the European tire label, mandatory on all tires sold in the Union, which rates rolling resistance and wet grip on a scale from A to E and reports external rolling noise in decibels, with a class from A to C. It is a useful yardstick but not an exhaustive one: it does not describe behaviour on dry roads, resistance to aquaplaning or mileage life.
Finally, there is maintenance. The legal tread depth limit is set at 1.6 mm, measured in the main grooves and signalled by the wear indicators, but wet performance degrades well before that threshold. Inflation pressure and the age of the tire, readable from the DOT code, should be checked regularly, since the compound ages even when no distance is covered. Replacement, moreover, should always be carried out at least in complete axle sets and with tires of the same size, type and level of wear: a difference in grip between the two sides of the same car is among the most treacherous conditions that can be created.