The formula
diameter = rim × 25.4 + 2 × (width × profile ÷ 100)
Where it comes from
The code 205/55 R16 says three things: 205 millimetres of tread width, a sidewall whose height is 55 % of that width, and a 16-inch rim. The overall diameter comes from adding the rim and the two sidewalls, and it is what has to stay the same for the car to keep measuring speed correctly.
How to work it out by hand
- Multiply the width by the profile and divide by a hundred: that is the sidewall height
- Convert the rim inches to millimetres by multiplying by 25.4
- Add the rim plus twice the sidewall
- Compare the diameters of the two sizes
What is worth knowing
The margin allowed in most regulations is 3 % of difference in diameter. Beyond that the speedometer drifts enough to matter: with a wheel larger than it should be, the car goes faster than the needle reads, which is the dangerous direction. It also affects ABS and stability control, which work out slip from wheel rotations.