How tire sizing works
Passenger tires are labeled with section width in millimeters, aspect ratio as a percentage of that width, and rim diameter in inches. Sidewall height in millimeters equals width × (aspect ÷ 100). Overall diameter is twice the sidewall plus the rim converted to millimeters (inch × 25.4). Circumference is π × diameter—this is what your wheel covers in one revolution.
What 205/55R16 means
205 is the tread width in millimeters. 55 means the sidewall is 55% of 205 mm (not 55 millimeters). R stands for radial construction, and 16 is the wheel diameter in inches. Same numbers on different cars can still load differently—always check load index and speed rating on the tire sidewall.
Why size changes affect the speedometer
Your speedometer infers road speed from how often the tire completes a revolution. A tire with a larger circumference travels farther each revolution. If you install a taller tire without re-calibrating, the vehicle may read slower than your true road speed (or the opposite for shorter tires, depending on what the vehicle was originally calibrated for). This tool treats Tire 1 as the reference the speedometer was accurate for.
Safe upgrade tips
Stay close to the original overall diameter to preserve gearing, ABS, and traction control behavior. Check clearance to fenders, struts, and brakes—especially when going wider. After a change, verify real speed with GPS on a safe road and consider professional speedometer recalibration if the offset is large.
Also explore: 205/55R16 vs 225/45R17, SUV tire comparison, example upgrade path.