Reading a tyre size
In 185/65 R15, 185 is the width in mm, 65 means the sidewall height is 65% of the width, R means radial, and 15 is the wheel diameter in inches.
Overall diameter = wheel × 25.4 + 2 × width × profile ÷ 100 (in mm). Keep a new size within about 3% of the original so the speedometer, ABS and gearing stay accurate.
Frequently asked questions
Will a bigger tyre make my speedometer wrong?
Yes. A taller tyre covers more ground per turn, so you go faster than the speedometer shows.