Decode and compare tire sizes. This free tire size calculator turns a size like 225/45R17 into overall diameter, sidewall and revs per mile, and compares two tires for diameter change and speedometer error.
Tire size
Tire size
mm
%
in
Tire 2 (new size)
mm
%
in
mph
Enter a tire size like 225/45R17 as width / aspect / wheel. Compare mode shows how a new size changes overall diameter and what your speedometer really means.
Overall diameter
—
Enter a tire size
Diameter
—
Sidewall height
—
Circumference
—
Revs per mile
—
Old diameter
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New diameter
—
Difference
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True speed
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Overall diameter = wheel diameter + twice the sidewall height, where sidewall = width × aspect ÷ 100. A larger overall diameter makes the speedometer read low (you’re actually going faster than it shows).
Use this free tire size calculator to find a tire’s overall diameter, sidewall height and circumference, and to compare two sizes for diameter change and speedometer error.
What this tire size calculator does
A tire size calculator turns a size like 225/45R17 into real dimensions — overall diameter, sidewall height, circumference and revolutions per mile — and can compare a new size against your current one.
So 225/45R17 has a 101.25 mm sidewall and a 24.97 in overall diameter (about 808 revolutions per mile).
Reading a tire size
Part
Example
Meaning
Width
225
Section width in mm
Aspect
45
Sidewall is 45% of width
Wheel
R17
17-inch wheel diameter
Tire size and your speedometer
Fitting a larger overall diameter makes the speedometer read low — you’re actually going faster than it shows. A smaller diameter does the opposite. Keep within about 3% of the original diameter to stay accurate.
How to use the tire size calculator
Enter width, aspect and wheel for your tire.
Switch to compare to add a second size.
Read the diameter change and true speed.
Tire Size FAQ
How do I read a tire size like 225/45R17?
225 is the width in mm, 45 is the aspect ratio (sidewall is 45% of width), and R17 is the 17-inch wheel diameter.
How do I calculate overall tire diameter?
Add the wheel diameter to twice the sidewall height: wheel(in)×25.4 + 2×(width×aspect÷100). For 225/45R17 that's about 24.97 inches.
How does tire size affect the speedometer?
A bigger overall diameter makes the speedometer read low — you're actually going faster than shown. A smaller diameter reads high.
How much bigger tire can I safely fit?
A common guideline is staying within about 3% of the original overall diameter to keep the speedometer and systems accurate. Always check clearance and your vehicle's specs.
What are revolutions per mile?
How many times the tire turns to cover one mile — useful for speedometer and gearing calculations. Smaller tires turn more times per mile.
Is the tire size calculator free?
Yes, this tire size calculator is completely free, needs no sign-up, and gives instant results in your browser.
