A tire size like 205/55R16 packs three numbers: width in millimetres, sidewall height as a percentage of width, and rim diameter in inches. From those you can get the overall diameter, which is what matters when you change wheels or tires: too different and the speedometer lies, the ABS misreads, and the tire may rub.
How it is calculated
- Sidewall height = width × aspect ratio ÷ 100. For 205/55: 205 × 0.55 = 112.75 mm.
- Overall diameter = rim diameter × 25.4 + 2 × sidewall. For R16: 406.4 + 225.5 = 631.9 mm (24.9 in).
- Circumference = π × diameter; revolutions per mile = 1,609,344 ÷ circumference.
- Speedometer error: the car counts wheel revolutions, so actual speed = indicated speed × (new diameter ÷ original diameter).
Example
Going from 205/55R16 to 225/45R17: original 631.9 mm, new 634.3 mm. Difference +0.4%, well inside the usual ±3% limit. At an indicated 60 mph you are actually doing 60.2. The wider tire (+20 mm) is the thing to check for clearance, not the diameter.
The 3% rule
Car makers calibrate the speedometer, ABS, traction control and gearing to the original diameter. A tire within about 3% keeps all of that within tolerance; beyond it, the speedometer is noticeably off and the electronics can misbehave. Bigger wheels normally need a lower aspect ratio to keep the diameter the same, which is the "plus sizing" rule of thumb: plus one inch of rim, minus about 10 points of aspect ratio.
Frequently asked questions
What do the numbers on a tire mean?
205/55R16: 205 mm wide, sidewall 55% of the width (113 mm), fits a 16-inch rim. The R means radial construction. A load index and speed rating usually follow, e.g. 91V.
Can I put a different size tire on my car?
Usually yes if the overall diameter stays within about 3% of the original and the width fits without rubbing. Check the load index is equal or higher and that the rim width suits the tire.
How much does a bigger tire affect the speedometer?
In proportion to the diameter change. A 3% larger tire makes you go 3% faster than shown: 62 at an indicated 60. The odometer under-counts distance by the same amount.
What is plus sizing?
Fitting a larger rim with a lower-profile tire so the overall diameter stays the same. Going from 16 to 17 inches, drop the aspect ratio by about 10 points, e.g. 205/55R16 to 215/45R17 or 225/45R17.
Sources
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked