How it is calculated
The load index and the speed index are not read here. The diameter is geometric, with no squat under the weight of the car.
Diameter = rim × 25,4 + 2 × width × profile / 100
Where the numbers come from
- Width — unit in the field: mm
- Profile — unit in the field: %
- Rim diameter — unit in the field: in
Example
205/55 R16: a sidewall of 112,75 mm, a diameter of about 631,9 mm.
Questions
Is a profile of 55 millimeters?
No, a percent of the width. 55% of 205 mm.
Is an R16 rim a radius?
In the marking, that is the bead-seat diameter in inches.
How much will the speedometer change?
Compare the rollout of the two sizes. The ratio of the lengths is about the ratio of the readings.