Compare two tire sizes and see the difference in diameter, width, and speedometer error. Decode any tire size into inches — diameter, sidewall, and more.
Pick a swap, then change the sizes to match your own tires.
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Thinking about a bigger wheel, plus-sizing, or wider off-road tires? The number on your sidewall — like 225/45R17 — controls more than looks: change it and you change your overall diameter, your speedometer reading, your ground clearance, and even your effective gearing. This calculator decodes any tire size into real inches and, in compare mode, puts your current tire next to a new one to show exactly how diameter, width, sidewall, and revolutions-per-mile change — plus the speedometer error the swap creates and whether it stays inside the safe ±3% window. Enter two sizes and get a side-by-side breakdown instantly, no signup required.
A tire size like 225/45R17 packs three numbers. The first, 225, is the section width in millimeters — the width of the tread from sidewall to sidewall. The second, 45, is the aspect ratio: the sidewall height as a percentage of the width, so here the sidewall is 45% of 225 mm, or about 101 mm. The 'R' means radial construction, and 17 is the wheel (rim) diameter in inches. To get the overall diameter, convert the rim to millimeters and add two sidewalls: 17 × 25.4 + 2 × 101.25 = about 634 mm, or 25.0 inches. From diameter you get circumference (π × diameter) and revolutions per mile (63,360 ÷ circumference in inches). When you fit a tire with a different overall diameter, your speedometer — calibrated for the original — reads off by the same percentage: a 2% larger tire makes an indicated 60 mph actually about 61.2 mph. The widely used safe limit is ±3% diameter change.
Tire Diameter & Speedometer Formula
Move to a bigger wheel with a lower-profile tire while keeping the overall diameter close to stock.
See how much taller a truck or SUV tire gets and the speedometer error it adds.
Already changed tire sizes? Find out how far off your speedometer and odometer now read.
Understand what 225/45R17 means and convert it to inches.
Confirm a different size still fits before you buy, by keeping diameter within ±3%.
Two shops offered different sizes? Compare the real dimensions side by side.
Compare two sizes side by side — diameter, width, sidewall, circumference, and revs per mile — not just one number.
A larger tire makes your speedometer read low. See exactly how far off it is before you fit a new size.
Get a clear safe-fitment verdict so you avoid clearance, gearing, and speedometer problems.
Turn 225/45R17 into inches — overall diameter, sidewall height, and section width.
Going up a wheel size? Keep the overall diameter close so the car drives and reads correctly.
Enter your sizes and get a full comparison immediately — no signup or sales pitch.
225 is the section width in millimeters, 45 is the aspect ratio (the sidewall is 45% of the width), R means radial, and 17 is the wheel diameter in inches. So 225/45R17 is a 225 mm-wide tire with a roughly 101 mm sidewall on a 17-inch wheel, giving an overall diameter of about 25.0 inches.
Your speedometer is calibrated for the original tire's overall diameter. A larger tire travels farther per revolution, so the speedometer reads low — you're actually going faster than it shows — and the odometer undercounts miles. A smaller tire does the opposite. The error equals the percentage change in overall diameter.
A common rule is to keep the overall diameter change within ±3% of the original. Inside that window the speedometer stays reasonably accurate and clearance is usually fine. Beyond it, you should check for rubbing, confirm load rating, and consider that gearing and speedometer accuracy are noticeably affected.
Plus-sizing means fitting a larger-diameter wheel with a lower-profile tire while keeping the overall tire diameter about the same. For example, 225/45R17 and 245/40R18 are close in overall diameter. It improves handling and looks without throwing off your speedometer.
Multiply the rim diameter by 25.4 to get millimeters, then add two sidewalls (each sidewall is width × aspect ÷ 100). For 225/45R17: 17 × 25.4 + 2 × (225 × 0.45) = 431.8 + 202.5 = 634.3 mm, which is about 25.0 inches. This calculator does it for you.
A wider tire (a bigger first number) needs enough wheel width and clearance in the wheel well and against suspension components. This calculator shows the section-width and diameter change, but always confirm clearance, wheel width, and load rating for your specific vehicle before fitting.
Revolutions per mile is how many times a tire turns to cover one mile, calculated as 63,360 (inches in a mile) divided by the tire's circumference in inches. A larger tire turns fewer times per mile, which is why it affects your odometer and speedometer.
No. You can compare any two sizes, including different rim diameters — that's exactly what plus-sizing involves. The calculator works from each tire's overall diameter, so a 17-inch and an 18-inch setup are compared fairly by their true dimensions.