Enter two tire sizes and compare sidewall height, overall diameter, circumference and revolutions per mile, plus what the size change does to the speedometer reading.
A tire size such as 225/65R17 means a 225 mm wide tread, a sidewall height equal to 65% of that width, and a 17 inch rim. Sidewall height is width multiplied by the aspect ratio; overall diameter is the rim diameter plus two sidewalls; circumference and revolutions per mile follow from the diameter. A common rule of thumb in tire sizing guides is to keep the overall diameter of a replacement size within about plus or minus 3% of the original, because larger changes alter gearing, ground clearance and the speedometer reading more noticeably. That 3% figure is guidance only, not a safety verdict: actual tire dimensions vary by manufacturer and model even at the same marked size, and the vehicle manufacturer and a tire fitter are the authorities on which sizes a specific vehicle can use.
What does a diameter difference mean? It is the percentage change in overall diameter between the two sizes. A positive figure means Tire B is taller overall, so it covers more ground per revolution and the vehicle travels faster than the speedometer indicates.
What is the plus or minus 3% rule of thumb? Many tire sizing guides suggest keeping overall diameter within about 3% of the original size. It is a rule of thumb for limiting speedometer and gearing changes, not a safety verdict, and it does not replace the vehicle manufacturer recommendations or a tire fitter advice for a specific vehicle.
Why does tire size change the speedometer reading? The speedometer infers speed from how fast the wheels turn, assuming the original tire circumference. A taller tire travels further per revolution, so the true speed is higher than the reading; a shorter tire does the opposite.