Tire Size Calculator
Enter a tire's section width, aspect ratio and rim diameter to find its overall height, circumference and revolutions per mile.
Overall tire diameter equals the rim diameter plus twice the sidewall height, where sidewall height is the section width times the aspect ratio. A 225/45R17 tire is about 24.97 inches tall: 17-inch rim + 2 ร (225 mm ร 0.45 รท 25.4). Comparing overall diameters tells you whether a new size will change your speedometer reading.
How to Use This Calculator
A tire size like 225/45R17 packs three numbers: 225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of the width), and 17 is the wheel diameter in inches. Enter those three values and the calculator returns the tire's overall diameter (its full height), its circumference, and how many times it turns in a mile.
The most common use is comparing two sizes before you buy. Calculate your current tire and a candidate replacement; if the overall diameters differ by more than about 3%, your speedometer, odometer and gearing will be noticeably off. Keeping the new diameter within a few percent of stock keeps everything reading correctly.
How the Calculation Works
The sidewall height in millimeters is the section width times the aspect ratio: 225 ร 45% = 101.25 mm. Divide by 25.4 to convert to inches (3.99 in). There are two sidewalls stacked on the wheel โ one at the top, one at the bottom โ so the overall diameter is the rim diameter plus twice the sidewall height. Circumference is ฯ times the diameter, and revolutions per mile is 63,360 inches per mile divided by the circumference in inches.
Worked Example
For a 225/45R17 tire: sidewall = 225 ร 0.45 = 101.25 mm = 3.986 in. Overall diameter = 17 + 2 ร 3.986 = 24.97 in. Circumference = ฯ ร 24.97 = 78.45 in. Revolutions per mile = 63,360 รท 78.45 = 808. Compare that to a 235/45R17: sidewall = 235 ร 0.45 = 105.75 mm = 4.163 in, diameter = 17 + 2 ร 4.163 = 25.33 in โ about 1.4% taller, small enough that the swap would barely change your speedometer.
What the Result Means
Overall diameter drives your speedometer accuracy and effective gearing. A taller tire makes the speedometer read low (you are actually going faster than it shows) and slightly lowers your effective axle ratio, dulling acceleration. A shorter tire does the opposite. Revolutions per mile is what electronic speedometers and many trip computers actually use, so matching revs/mile to the original tire keeps those systems accurate. Section width affects grip and clearance but not speedometer calibration.
Assumptions and Limitations
This uses the nominal P-metric geometry printed on the sidewall. Real mounted tires vary slightly from the calculated size depending on the actual rim width, inflation pressure and load, and manufacturers are allowed a tolerance, so two tires of the same printed size can differ by a few millimeters. The revs-per-mile figure uses the unloaded diameter; under load the rolling diameter is a little smaller, so real revs/mile run slightly higher. Use the result for comparison and speedometer estimates, not for legal or safety certification.
When to Use a Tire Size Calculator
Car owners and enthusiasts use this to convert a P-metric tire size into overall diameter, circumference, and revolutions per mile. Run it before changing tire size so you can see how it affects your speedometer and gearing. Compare two sizes to keep the diameter within a safe few percent of stock.
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Open calculator โFrequently Asked Questions
What do the numbers in a tire size mean?
In 225/45R17: 225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percent of width), R means radial construction, and 17 is the wheel diameter in inches.
How much can I change tire size safely?
As a rule of thumb, keep the overall diameter within about 3% of the original so your speedometer, odometer and traction systems stay accurate. Also confirm the new size clears your suspension and fenders.
Will a bigger tire mess up my speedometer?
Yes. A taller tire travels farther per revolution, so the speedometer reads lower than your true speed. A 3% larger diameter makes an indicated 60 mph actually about 62 mph.
What is aspect ratio on a tire?
It is the sidewall height expressed as a percentage of the section width. A 45 aspect ratio means the sidewall is 45% as tall as the tire is wide. Lower numbers mean shorter, sportier sidewalls.
How do I find revolutions per mile?
Divide 63,360 (inches in a mile) by the tire's circumference in inches. The calculator does this automatically once you enter width, aspect ratio and rim size.
Does a wider tire have a bigger diameter?
Not necessarily. Width and diameter are independent โ a wider tire with a lower aspect ratio can be the same height. Always compare the calculated overall diameters, not just the width numbers.
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