Tire Size Calculator
Calculate and compare tire sizes, dimensions, and speedometer differences
Original Tire Size
225/45R17
Dimensions
Overall Diameter24.97" / 634.3 mm
Sidewall Height101.3 mm
Circumference78.45" / 1992.7 mm
Revolutions/Mile808
Revolutions/Km502
New Tire Size
235/40R18
Dimensions
Overall Diameter25.40" / 645.2 mm
Sidewall Height94.0 mm
Circumference79.80" / 2027.0 mm
Revolutions/Mile794
Revolutions/Km493
Size Comparison
Diameter Difference+10.9 mm (+1.72%)
Clearance Difference+5.5 mm
The new tire is larger. Your speedometer will read slower than actual speed.
Speedometer Error
| Speedometer Shows | Actual Speed | Difference |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 mph |
| 50 mph | 50.9 mph | +0.9 mph |
| 70 mph | 71.2 mph | +1.2 mph |
| 100 mph | 101.7 mph | +1.7 mph |
Note: Most manufacturers recommend staying within 3% of the original tire diameter.
Visual Comparison
Original
225/45R17
24.97" diameter
New
235/40R18
25.40" diameter
Understanding Tire Sizes
Tire sizes are expressed in the format Width/Aspect Ratio R Rim Diameter (e.g., 225/45R17).
- Width (225): The width of the tire in millimeters from sidewall to sidewall.
- Aspect Ratio (45): The height of the sidewall as a percentage of the width. A 45 means the sidewall is 45% of 225mm = 101.25mm.
- Rim Diameter (17): The diameter of the wheel rim in inches.
- R: Indicates radial construction (most common type).
When changing tire sizes, its important to stay within 3% of the original overall diameter to maintain accurate speedometer readings and proper vehicle handling.