Tyre sizes for passenger cars are usually expressed as something like 205/65 R15 94V.
This is how you read the numbers:
205 refers to the width of the tyre (sidewall to sidewall) in mm
65 refers to the ratio of the sidewall, to the tread. For example in this case the sidewall is 65% of 205. Check some tyres that have an aspect ratio of 35, and you’ll see the difference.
R means that the tyre is a radial ply construction. All passenger tyres these days are “R”, which means that the casing cords run radially from bead to bead and belt cords run directly under the tread.
15 refers to the rim diameter in inches
The 94 refers to the load index (see below).
The letter appearing after the load index is the speed rating (see below).
Here’s what the load index number means:
81 = 462 kg
82 = 475 kg
85 = 515 kg
86 = 530 kg
87 = 545 kg
90 = 900 kg
91 = 630 kg
94 = 670kg
95 = 690 kg
100 = 800 kg
Here’s what the speed rating symbol means:
N = 140 km/h
P = 150 km/h
Q = 160 km/h
R = 170 km/h
S = 180 km/h
T = 190 km/h
U = 200 km/h
H = 210 km/h
V = 240 km/h
Z = 240 km/h
W = 270 km/h
Y = 300 km/h
Note 1: Yes, some of them seem to be out of order, and some are the same!
Note 2: These are maximum tested speeds – not recommended speeds!
Do not mix and match tyres with different speed ratings on your vehicle. Speed rated tyres should be replaced in sets of 4, and there’s not problem in using a higher speed rating tyre on your vehicle than recommended. Just don’t go lower than the values listed on your tyre specification from your car manufacturer. Consult your car manual.