Free Tire Size Calculator
Enter your stock tire size and the size you are considering. See the diameter difference and what it does to your speedometer.
This free tire size calculator decodes sizes like 265/70R17 (265 mm wide, sidewall 70 percent of width, 17-inch rim) and computes sidewall height, overall diameter, circumference, and revolutions per mile. A 265/70R17 measures 31.6 inches in diameter; switching to 285/70R17 grows it to 32.7 inches, a 3.5 percent increase that makes the speedometer read 60 mph while you actually travel 62.1 mph.
For fitment, confirm clearance, load rating, and TPMS compatibility with a tire professional. Diameter changes affect gearing, ABS calibration, and odometer readings.
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How tire size math works
A size like 265/70R17 packs three numbers: 265 is the tread width in millimeters, 70 is the aspect ratio (sidewall height as a percent of width), and 17 is the rim diameter in inches. The R just means radial construction.
Sidewall height is width times aspect ratio divided by 100, converted to inches by dividing by 25.4. For 265/70R17: 265 times 0.70 is 185.5 mm, divided by 25.4 is 7.3 inches of sidewall. Overall diameter is the rim plus two sidewalls: 17 + 14.6 = 31.6 inches.
Circumference is pi times diameter: about 99.3 inches for our example. Revolutions per mile is 63,360 (inches per mile) divided by circumference: about 638. These two numbers drive the speedometer math.
Compare with 285/70R17: sidewall 7.9 inches, diameter 32.7 inches. The diameter grows 3.5 percent, so the tire covers 3.5 percent more ground per revolution. Your speedometer, calibrated for the old size, reads low: at an indicated 60 mph you are actually doing 62.1 mph.
The rule of thumb is to stay within 3 percent of stock diameter to keep the speedometer, odometer, gearing, and ABS happy. Beyond that, check clearance at full steering lock and full suspension compression, verify the load rating meets your vehicle, and consider recalibrating the speedometer.
Tire size calculator questions
How do I read my tire size?
The format is width/aspect R rim. In 265/70R17, 265 is the tread width in millimeters, 70 means the sidewall is 70 percent of the width, R means radial, and 17 is the wheel diameter in inches. The full size is usually molded on the tire sidewall and listed on the driver-door placard.
Will bigger tires mess up my speedometer?
Yes. The speedometer counts revolutions, so a larger-diameter tire travels farther per revolution and the gauge reads low. A 3.5 percent diameter increase, like 265/70R17 to 285/70R17, turns an indicated 60 mph into 62.1 actual. Stay within 3 percent of stock to keep error small.
What is the 3% rule for tire sizes?
It is the widely used guideline: replacement tires should stay within 3 percent of the original overall diameter. Inside that band, speedometer error, gearing changes, and clearance risks stay small. Beyond it, verify fitment carefully and consider recalibration.
Do I need new wheels for bigger tires?
Only if you change the rim diameter number. A 285/70R17 fits the same 17-inch wheels as a 265/70R17, as long as the wheel width suits the wider tire. Changing from 17 to 18 inch rims does require new wheels.