Tire Size Converter

A tire size like 225/45R17 means:

• 225 = tread width in millimeters
• 45 = aspect ratio (sidewall height as % of width)
• R = radial construction
• 17 = wheel (rim) diameter in inches

Calculating overall tire diameter:

1. Sidewall height = width × (aspect ratio / 100)
2. Sidewall height in inches = sidewall height (mm) ÷ 25.4
3. Overall diameter = (2 × sidewall in inches) + rim diameter

Example for 225/45R17:

• Sidewall = 225 × 0.45 = 101.25 mm = 3.986 inches
• Overall diameter = (2 × 3.986) + 17 = 24.97 inches (634 mm)
• Circumference = diameter × π = 78.44 inches (1,992 mm)

Revolutions per mile:

• Revs/mile = 63,360 ÷ circumference (inches)
• Revs/km = 1,000,000 ÷ circumference (mm)

Common tire sizes:

The reason diameter is the number to watch is your speedometer. It’s calibrated for the original tire’s circumference, so fitting a taller tire makes the wheels turn slower for the same speed and the speedo reads low, while a shorter tire reads high. The rule of thumb is to keep overall diameter within about 3% of stock, or your speed, odometer, and even some safety systems start drifting off. That’s the whole point of comparing the old and new size before you buy.

This is also the logic behind “plus-sizing.” When people fit bigger rims, they pair them with a lower aspect ratio so the overall diameter stays the same: a 17-inch wheel with tall sidewalls and an 18-inch wheel with shorter ones can roll out to identical heights. Two things the size string doesn’t show are worth knowing: the aspect ratio drives ride feel, since a low-profile tire looks sharp and handles crisply but transmits every pothole, and the separate load and speed rating stamped on the sidewall must still meet what your car needs, whether or not the dimensions match.