Tire Size Calculator — Compare & Convert
Decode tire size numbers. Compare two tire sizes for speedometer accuracy, clearance, and ride differences.
Decoding Tire Size Numbers
A tire marked 225/45R17 means: 225mm tread width, 45% aspect ratio (sidewall height is 45% of width = 101mm), R = radial construction, 17 = rim diameter in inches. The overall tire diameter is the rim diameter plus twice the sidewall height. This number determines speedometer accuracy, ground clearance, and wheel well fitment.
Safe Size Changes
When changing tire sizes, keep the overall diameter within 3% of original. Beyond 3%, your speedometer reads inaccurately, ABS and traction control may malfunction, and odometer readings will be off (affecting lease mileage tracking and resale value). A common upgrade path: moving up one rim size (17 to 18) while decreasing the aspect ratio to maintain the same overall diameter.
Wider tires do NOT always improve grip. On wet or snowy roads, narrower tires often perform better because they concentrate weight over a smaller contact patch, cutting through water and snow more effectively. The optimal tire width depends on driving conditions, not just looks.
