Was first system to be introduced where the tires cross-section and rim diameter are usually indicated. Example:

7.25-15 means:

7.25 is the cross section width of the tire in inches
15 are the rim diameter in inches.

Note: If the cross section width ends in zero like 7.0 instead of 7.25, it means the tire is commonly high profile. Therefore 7.25 are low profile tire.

This system is an improvement of the first system where the load carrying capacity of the tires is added. Example:

FR80-15 means:

F refers to the load rating of the tires
R means it is radial constructed
80 is still the cross section
15 are still the rim diameter.

Widely used my most manufacturers, this system were introduced to accommodate smaller or compact cars. Letter “P” designation is use which refers to passenger car tire. An example is shown below:

P215/65R15 where:

P is for passenger type car tire (not truck)
215 is section width in millimetres
65 is aspect ratio
R is for radials
15 is the rim diameter in inches

Primarily used in Europe and is equivalent to the Numeric system above except the cross section measurements is done in metric. Example:

195/60R14

195 is the section width in millimetre
60 is the aspect ratio
R is radial construction
14 is rim diameter in inches

With the introduction of International Standard Organization (ISO), the metric system was combined with service description. Example:

195/60R14 82H

The 82H is the additional designation, which means:
82 is the load index
H is the rated speed

This system is the same as the Metric system except all measurements are done in millimetre.

This system is based on the Numeric system above except it is used for trucks and a light truck (LT) designation is added. This is used primarily in commercial vehicles. Example:

9.50R16.5LT/D where:

9.50 is section width in inches
R is radial construction
16.5 is rim diameter in inches
LT is light truck designation
D is the load range

This system mirrors the P-Metric for passenger cars except it is used for light trucks. Example:

LT235/75R15C

LT is light truck designation
235 is section width in millimetres
75 is aspect ratio
R is radial construction
15 is rim diameter in inches
C is the load range

To get better on watery off road situations, this system was introduced with a lower aspect ratio and high floatation. Example:

31X10.50R15LT/C

31 is tire overall diameter in inches
10.50 is section width in inches
R is radial construction
15 is rim diameter in inches
LT is light truck designation
C is load range

Notes:

Aspect ratio is the dimensional relationship of the tire’s section height to the section width, which is expressed in percentage. The lower the aspect ratio, the shorter the length of the sidewall. This results in quicker steering response.

Load index is the assigned number ranging from 0 to 279, which refers to the load carrying capacity of the tire. Most cars use 75 to 105 as regular range for its application. This load index is expressed in pounds or kilograms load at maximum inflation pressure.