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TIRE SIZING
A number of tire sizing systems are currently in place in both passenger car and light truck tire applications.
In terms of passenger car tire sizing, six systems exist. The systems include the P-metric, the alphanumeric, the millimetric, the European metric, the ISO metric and the numeric.
1. P-metric. The P-metric system is the most widely used in North America on the vast majority of passenger performance tires. Developed in the 1970s in an attempt to standardize tire sizing, this system uses a combination of metric and inch designations. The "P" simply indicates that the tire is designed for passenger car use. The first number represents the tire section width in millimeters. This is followed by the aspect ratio percentage figure (the relationship of the tire´s section height to its section width), while the rim diameter is denoted in inches. For example: a P245/50R16 designation means that this passenger tire has a section width of 245mm and an aspect ratio of 50 (the tire section height measures 50% of the section width). The R means that the tire is of radial construction. The 16 indicates that the required rim diameter is 16 inches.
2. European metric. This system is essentially the same, except the aspect ratio may or may not be indicated (if the tire has an 82 aspect ratio, it may not be marked. If the tire has an aspect ratio other than 82, then it would be included in the marking). The label includes the section width in millimeters, the speed rating letter, an "R" if it´s a radial tire, and the rim diameter. Oddly enough, even though this is a so-called "metric" tire, the rim diameter is indicated in inches. For example, a 175SR14 indicates a radial tire with a 175mm section width, an "S" speed rating, and requiring a 14-inch diameter rim. If the aspect ratio is indicated, that number will follow the section width number (such as a 195/70SR14).
3. Millimetric. An offshoot of the European metric system is the millimetric system, in which both the section width and the rim diameter is identified in millimeters instead of inches. For example, a 255/50VR390 indicates a tire with a section width of 255mm, a 50-series aspect ratio, a V speed rating, radial construction, and requiring a rim diameter of 390mm.
4. ISO (International Standards Organization) metric. With ISO metric sizing, the tire´s load rating is added to the main size label, which follows the normal metric sizing system rules. For example, a 185/60R14 82H indicates a tire with a section width of 185mm, a 60-series aspect ratio, radial construction, requires a 14-inch rim, a load index of 82 and a speed rating of H.
5. Numeric. The oldest of the six systems, it is very basic. The label refers to the tire´s section width and rim diameter only. A 8.25-14 indicates a tire with a section width of 8.25 inches and designed to mount to a 14-inch wheel. This system was used back when there were not a lot of aspect ratio choices. Basically, there were two common aspect ratios: 92 and 82. If the section width number ended in zero, then assume that the tire had an aspect ratio of about 92. If the section width number ended in a number other than zero, then assume that the aspect ratio was about 82. For instance, a 7.00-14 had an aspect ratio of about 92; while a 7.75-14 had an aspect ratio of about 82. The actual aspect ratios could vary wildly, as each size/model of tire could have an aspect ratio anywhere in the approximate range of the 92 or 82 index. This system is important when dealing with vintage tires.
6. Alphanumeric. This tire sizing system was used in the 1960s, and remains applicable today for folks who collect and restore many domestic muscle cars (when the car owner wants to make every effort to follow all original specs and details, he/she will locate new replicas or retreaded versions of the original tire brands, models and sizes). This system used a letter to designate the tire load and one aspect of its overall size. The aspect ratio indicated the relationship between section height and width.
For instance, a GR60-15 indicated a tire with a "G" load and height rating, radial construction ("R"), a 60 aspect ratio, and a required rim diameter of 15 inches. The first letter (the load/size rating) ranged from A through N. The higher the letter, the bigger the tire and the more load it could handle. The now classic 1969 Camaro Z28, and similar-vintage cars like the Mustang, GTO, Dart, Barracuda and others, were all equipped with alpha-numeric tire sizes. For example, the 1968-72 Corvettes were equipped with F70-15 nylon belted tires; and in 1973 were outfitted with GR70-15 radials. It wasn´t until 1978 that the ´Vette was shod with P-metric tires (P255/60R15 or P225/70R15 radials). In 1984, the Corvette switched rim size, using a P255/50VR16. In 1989 a P275/40ZR17 was used, and in 1993 the Corvette started using dual-sized systems, with a P255/45ZR17 up front and a P285/40ZR17 in the rear. The 1997 version switched to P275/40ZR18 rear tires.
wur10 hat geschrieben:Punkt 6
The first letter (the load/size rating) ranged from A through N. The higher the letter, the bigger the tire and the more load it could handle
For instance, a GR60-15 indicated a tire with a "G" load and height rating, radial construction ("R"), a 60 aspect ratio,
ich hab 14" mit einem längenverhältnis von 60 
(dürft das verhältnis lauffläche zu höhe sein)
b) Alphanumerisches System, z.B. FR70x14
Je näher der erste Buchstabe am Anfang des Alphabets, desto schmaler ist der Reifen, der zweite Buchstabe R steht ggf. für einen Radial-Reifen, die folgende zweistellige Zahl gibt das Höhen/Seitenverhältnis an, die letzten beiden wieder die Felgenhöhe. Beim FR70x14 handelt es sich also in etwa um einen 205/70R14, E stünde dann für circa 195, G circa für 215 als grobe Faustformel. Auch hier ist aber für eine Übersetzung ins metrische der Durchmesser bzw. Abrollumfang ausschlaggebend
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