Highlander:
Why are vehicle tyre sizes a mixture of imperial and metric?
I often wondered this and discovered many years ago what each tyre number meant. Here is a shortened explanation of 4 popular sizes.
Example 1 = 205/65R15 94H (Passenger Car tyre size)
- Size Designation: 205/65R15
205 - Section Width in mm
65 - Aspect Ratio
R - indicates radial casing construction
15 - Nominal Rim Diameter in inches
94 - Load Index (94 - 670kg)
H - Speed Symbol (H - 210kph).
Millimetric
Example: 180/65R390 85V (Passenger Car tyre size)
- Size Designation: “180/65R390”
180 - Section Width in mm
65 - Aspect Ratio
R - indicates radial casing construction
390 - Nominal Rim Diameter in mm
85 - Load Index (85 - 515kg)
V - Speed Symbol (V - 240kph).
Example A: 8.25R16 128/126L (Light Truck tyre size)
- Size Designation: 8.25R16
8.25 - Section Width in inches
R - indicates radial casing construction.
16 - Nominal Rim Diameter in inches.
128 - Load Index when fitted as a single tyre (128 - 1800kg)
126 - Load Index when fitted as one tyre in a dual tyre set (126 - 1700kg)
L - Speed Symbol (L - 120kph)
Example B: 10.00 R20 16
- Size Designation: “10.00R20”
10.00 - Section Width in inches
R - indicates radial casing construction
20 - Nominal Rim Diameter in inches.
It doesn’t explain why they use both measurements, but it explains what the number and letters mean.