How to read a tire size
In 205/50R15, 205 is the tread width in millimeters, 50 is the sidewall height as a percent of the width (the aspect ratio), and 15 is the wheel diameter in inches. So the sidewall is 205 × 0.50 = 102.5 mm tall, and the whole tire is 2 × 102.5 mm + 15 in ≈ 23.1 in across.
The ±3% rule
Most enthusiasts keep a new size within about 3% of stock diameter. Past that, the speedometer and odometer drift further, ABS and traction control can get confused on some cars, and the tire is more likely to rub at full lock or full compression. It's a rule of thumb, not a law: always check real clearance on your car.
Speedometer error
A taller tire covers more ground per turn, so you're going faster than the speedometer shows. A shorter tire does the opposite. The calculator assumes your speedometer was accurate on the current size; most factory speedometers already read a little high.
Plus-sizing
Going up an inch in wheel size ("plus one") usually means about 10 mm more width and 5–10 points less aspect ratio to keep the same diameter, for example 205/50R15 → 215/45R16. The shorter sidewall sharpens steering response but rides harsher and gives the wheel less protection from potholes.