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 ShowsActual SpeedDifference
30 mph30.5 mph+0.5 mph
50 mph50.9 mph+0.9 mph
70 mph71.2 mph+1.2 mph
100 mph101.7 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.