How to use this tire size calculator
- Find your current tire size on the tire sidewall or on the placard inside the driver's door jamb.
- Enter it as the original size, either metric (such as 265/70R17) or flotation (such as 35x12.50R17).
- Enter the size you're thinking of switching to.
- Compare diameter, sidewall, circumference and revolutions per mile for both sizes.
- Check the diameter difference and the speedometer error before you buy.
How it's calculated
For a metric size, the sidewall height in millimeters is the width × the aspect ratio ÷ 100. The overall diameter is two sidewalls plus the wheel: 2 × sidewall + rim diameter × 25.4. The circumference is π × diameter, and revolutions per mile = 1,609,344 ÷ circumference in millimeters, since a mile is 1,609,344 mm. Flotation sizes state the overall diameter directly in inches, so 35x12.50R17 is a 35-inch tire, 12.5 inches wide, on a 17-inch wheel.
To compare sizes, the calculator finds the percentage difference in diameter. Speedometer error follows from that: actual speed = indicated speed × new diameter ÷ old diameter.
Example: a 265/70R17 has a sidewall of 185.5 mm and a diameter of 802.8 mm (31.6 in). It turns about 638 times per mile. A 285/70R17 has a 199.5 mm sidewall and an 830.8 mm (32.7 in) diameter, turning about 617 times per mile. That's 3.5% larger, so when your speedometer shows 60 mph, you're actually doing about 62.1 mph. Jumping to a 35-inch tire (889 mm) is 10.7% larger than the original, which turns an indicated 60 mph into about 66.4 mph.
Choosing a new tire size
- Stay within about 3% of the original diameter unless you plan to recalibrate the speedometer.
- Check the load index and speed rating. A replacement should at least match the ratings on your door placard.
- Confirm clearance at full steering lock and full suspension compression, not just when the vehicle is parked.
- Expect taller tires to reduce acceleration and fuel economy slightly, and possibly change how your transmission shifts.
- Replace tires in matched sets on all-wheel-drive vehicles, where different diameters can strain the drivetrain.
- Keep the inflation pressure your vehicle maker recommends, not the maximum printed on the sidewall.
Good to know: These are nominal dimensions. Real tires of the same size vary slightly by brand and tread depth, and the diameter shrinks a little as the tread wears.
Frequently asked questions
What do the numbers in a tire size like 265/70R17 mean?
265 is the tread width in millimeters, 70 is the aspect ratio (sidewall height as a percentage of width), R means radial construction and 17 is the wheel diameter in inches. Taller or wider tires change diameter, which affects your speedometer.
How much bigger can I go on tire size?
A common guideline is to stay within about 3% of the original overall diameter. Bigger changes can affect speedometer accuracy, transmission shift points, stability systems and clearance, so larger jumps usually need recalibration and sometimes a lift or different wheels.
Will bigger tires make my speedometer read wrong?
Yes. A larger tire covers more ground per turn, so you travel faster than the speedometer shows. A smaller tire has the opposite effect. Your odometer will be off by the same percentage.