Enter two tire sizes in standard format (e.g. 225/45R17) to compare overall diameter, circumference, revolutions per mile, and speedometer difference. Useful when upsizing rims, switching to winter tires, or checking whether a replacement size keeps your odometer reading accurate.
A speedometer difference over 3% is noticeable. Over 5% may affect accuracy enough to matter for speed readings.
A tire size like 225/45R17 has three parts: section width in mm, aspect ratio as a percentage of width, and rim diameter in inches. The "R" indicates radial construction, which is standard on virtually all modern passenger car tires.
Sidewall height = (aspect ratio / 100) x section width. For 225/45R17, the sidewall is (45/100) x 225 = 101.25 mm = 3.99 inches. Overall diameter = rim diameter + (2 x sidewall height in inches) = 17 + (2 x 3.99) = 24.98 inches.
Your speedometer reads speed from wheel rotation speed. A larger diameter tire completes fewer rotations per mile, so the sensor sees fewer pulses and the displayed speed reads low. If your new tire has a 2% larger diameter, your speedometer reads 58.8 mph when you are actually doing 60. This calculator shows you that real-world speed so you know what to expect before the tires go on.
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The three numbers mean: section width in millimeters (225), aspect ratio as a percentage of section width (45), and rim diameter in inches (17). A 225/45R17 tire is 225 mm wide, the sidewall height is 45% of 225 mm = 101.25 mm, and it fits a 17-inch rim. The "R" stands for radial, which is the construction method used on all modern passenger tires.
Overall diameter = (sidewall height x 2) + rim diameter in inches. First calculate sidewall height in mm as (aspect ratio / 100) x section width, then convert to inches by dividing by 25.4. For 225/45R17: sidewall = (45/100) x 225 = 101.25 mm = 3.986 in. Diameter = (3.986 x 2) + 17 = 24.97 inches.
Yes. The speedometer reads speed based on wheel rotations per unit of time. A larger overall diameter means fewer rotations per mile, so the speedometer reads low and your actual speed is higher than displayed. A smaller diameter gives more rotations per mile, making the speedometer read high. The percentage difference equals ((new diameter minus old diameter) / old diameter) x 100.
Most vehicle manufacturers allow plus or minus 1 to 3 percent. A difference over 3 percent is noticeable in daily driving. Over 5 percent is significant enough that you can receive a speeding citation while your dashboard shows a legal speed. The comparison section of this calculator color-codes the result: green for under 2%, yellow for 2 to 4%, red for over 4%.
Plus sizing means fitting a larger rim diameter with a lower-profile tire to keep the overall diameter close to stock. Going from 17 to 18 inches typically means reducing the aspect ratio by about 5 points. Going from 17 to 20 inches may require dropping the aspect ratio by 15 to 20 points. Enter both sizes in the calculator to confirm the diameter stays within 1 to 2 percent of stock.
Width alone does not affect speedometer accuracy because the speedometer counts rotations, not width. Only the overall diameter matters. However, a wider tire at the same aspect ratio is also taller, which does change the diameter. Always enter the complete tire size to get the correct diameter calculation.
Width in inches = section width / 25.4. Sidewall height in inches = (aspect ratio / 100) x section width / 25.4. Overall diameter = rim diameter + (2 x sidewall height in inches). This calculator performs all three conversions automatically when you enter a size in the standard format.