Tire Size Calculator
Compare tire sizes online free: diameter, sidewall, width, revolutions per mile and speedometer error. Free online tire size calculator, metric and flotation.
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How to use Tire Size Calculator
- 1Type your current tire size from the sidewall, for example 225/45R17 (flotation sizes like 33x12.50R15 also work)
- 2Type the new size you are considering to compare them side by side
- 3Check the overall diameter difference (stay within ±3%) and see how fast you really drive when your speedometer shows a given speed
Frequently asked questions
How do I calculate tire diameter?▾
Diameter = rim diameter + 2 × sidewall height, where sidewall = width × aspect ratio ÷ 100. For 225/45R17: sidewall = 225 × 0.45 = 101.25 mm, so diameter = 17 × 25.4 + 2 × 101.25 = 634.3 mm (24.97 inches).
How much tire size difference is safe?▾
Most tire and vehicle makers recommend keeping the overall diameter within about 3% of the original. Larger differences can confuse the speedometer, ABS and stability control, change gearing and cause rubbing.
How do bigger tires affect my speedometer?▾
A taller tire covers more distance per turn, so you drive faster than the speedometer shows. If the new tire is 3% larger, an indicated 60 mph is really about 61.8 mph.
What does 225/45R17 mean?▾
225 is the tread width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction and 17 is the wheel diameter in inches.
What is plus sizing?▾
Plus sizing means fitting a larger wheel (for example 17 to 18 inches) with a lower-profile tire so the overall diameter stays nearly the same — like 225/45R17 to 235/40R18, which differs by only about 1.7%.
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