How it works
Tire diameter is (width × aspect ÷ 50) + (rim × 25.4). Changing the spec shifts the real diameter, which throws off your speedometer reading — staying within ±3% is generally considered safe.
FAQ
Q. What goes wrong if the error is large?
A. Beyond speedometer/odometer drift, larger errors raise the risk of suspension clearance issues and ABS/ESC misreads.
Q. Why is it hard to match diameter exactly?
A. Width, aspect, and rim usually only come in steps of 5, so you're matching the closest available combination rather than a perfect number.