How tire size is calculated
A tire like 225/45 R17 means a 225 mm tread width, a sidewall that's 45% of that width (the aspect ratio), mounted on a 17-inch wheel. The overall diameter is the wheel diameter plus two sidewalls:
diameter = (wheel × 25.4) + 2 × (width × aspect ÷ 100)
Change the diameter and you change your true speed at any given speedometer reading, your effective gearing, and how much rubber sits inside the arch. That's why fitment guides push the ±3% rule — stay inside it and the car behaves the way the factory intended.
FAQ
Why does a bigger tire make my speedometer wrong?
Your speedometer counts wheel revolutions and assumes the factory tire's circumference. A taller tire travels further per revolution, so you're actually going faster than the dial shows — the speedo under-reads. A shorter tire does the opposite.
How much tire size change is safe?
As a rule of thumb, keep the overall diameter within ±3% of the original. Beyond that you'll notice speedometer error, altered gearing, and a higher chance of rubbing on suspension or bodywork at full lock or compression.
Is a "plus size" the same diameter?
Done properly, yes. Plus-sizing goes up in wheel diameter while dropping the aspect ratio to keep the overall diameter nearly identical — bigger wheel, shorter sidewall, same rolling circumference.