Tire Size Calculator
Enter the two tire sizes you want to compare (format: width/aspect ratio R rim diameter, e.g. 225/45R17). See the overall diameter, sidewall height, and how much a size change would throw off your speedometer.
Current Tire
New Tire
How to compare two tire sizes
Enter width in millimeters, aspect ratio as a percentage and wheel diameter in inches for both tires. For 225/45R17, enter 225, 45 and 17. The calculator compares overall diameter and sidewall height.
Tire diameter formula and example
Diameter in millimeters = wheel diameter × 25.4 + 2 × tire width × aspect ratio ÷ 100. A 225/45R17 tire has a nominal diameter of 634.3 mm (24.97 inches). A 235/40R18 tire measures 645.2 mm (25.40 inches), an increase of about 1.72%.
Tire size calculator questions
What happens to speedometer readings with larger tires?
With a larger rolling diameter, actual road speed is higher for the same indicated speed. In the example above, an indicated 60 mph corresponds to approximately 61.0 mph.
Does matching diameter guarantee fitment?
No. This tool does not check wheel offset, rim width, suspension clearance, load rating or manufacturer approval. Check those separately.
Are these measured tire dimensions?
No. These are nominal dimensions calculated from the size label. Actual dimensions vary with tire construction, load and pressure.