Tire Size Calculator
Decode a tire size like 225/45R17 into overall diameter, sidewall height, circumference, and revolutions per mile. Add a second size to see the speedometer error.
Original tire
How to use this calculator
1. Enter your tire code
Type a size like 225/45R17 into the code box and the width, aspect ratio, and rim fields fill in automatically. You can also skip the code and type the three numbers directly.
2. Read the dimensions
The result shows overall diameter (in inches and millimetres), the sidewall height, the rolling circumference, and how many revolutions the tire makes per mile.
3. Compare a replacement size
Enter a second code or set of fields to see the percentage difference in diameter and how it would skew a speedometer calibrated for the original tire.
4. Keep the diameter close
For plus-sizing, most guides keep the overall diameter within about 3% of the original so the speedometer, odometer, and electronics stay accurate.
About tire sizes
How do I read a tire code?
In 225/45R17, 225 is the tread width in mm, 45 is the aspect ratio (sidewall height as a percent of width), R is radial construction, and 17 is the rim diameter in inches. Sidewall = width × aspect ÷ 100; overall diameter = rim + two sidewalls.
Why diameter matters
Revolutions per mile = 63,360 ÷ circumference (in inches). A taller tire turns fewer times per mile, which changes how distance and speed are measured and can affect gearing and ground clearance.
Speedometer and odometer error
The gauges assume the original diameter. A larger tire makes the speedometer read low (you are actually going faster); a smaller tire reads high. The comparison shows your true speed when the dial reads 60 mph.