Tyre Size Comparison Calculator

The formula

A size like 205/55R16 gives the width in millimeters (205), the aspect ratio (55, the sidewall height as a percentage of the width) and the rim diameter in inches (16). Sidewall height = width × aspect ÷ 100. Overall diameter = rim × 25.4 + 2 × sidewall. Circumference = π × diameter, and revolutions per km = 1,000,000 ÷ circumference in mm.

The speedometer is set up for the original tyres, so when the new overall diameter is larger by a ratio, you travel that much faster than it shows: true speed = displayed speed × (new diameter ÷ old diameter).

Worked example

205/55R16: sidewall = 205 × 55 ÷ 100 = 112.75 mm, diameter = 16 × 25.4 + 2 × 112.75 = 631.9 mm, circumference = 1985 mm. 225/45R17: sidewall = 101.25 mm, diameter = 17 × 25.4 + 2 × 101.25 = 634.3 mm, circumference = 1993 mm. The new tyre is 2.4 mm (0.38%) taller, so with the speedometer showing 100 the true speed is 100 × 634.3 ÷ 631.9 = 100.4.

Reading the result

• A difference of a percent or two in overall diameter keeps the gearing and speedometer close to the original; larger differences matter more. The limit for your vehicle is set by the manufacturer, not by this calculator.
• A larger diameter means the car covers more ground per revolution: the speedometer and odometer read low, and gearing feels slightly taller.
• A smaller diameter does the opposite, so the speedometer reads high.
• The sidewall figure shows how much rubber there is between the rim and the road. A smaller aspect ratio on a bigger rim, as in the example, gives a shorter sidewall with a similar overall diameter.

Limits

• It uses nominal dimensions only. A tyre's real diameter varies with the brand, tread wear, inflation and load, and its real section width depends on the rim width.
• It cannot tell you whether the tyre fits inside the wheel arch, clears the brakes and suspension, matches the load or speed rating, or is allowed for your vehicle.
• It assumes the original speedometer was reading correctly with the current tyres. Many speedometers are built to read slightly high, so the true speed may already be a little below the display.
• The text boxes accept sizes with R, ZR or a dash before the rim size, and widths from 125 to 445 mm, aspect ratios from 20 to 95 and rims from 10 to 30 inches. Size markings for trailers, motorcycles and load-index codes are not read.

More worked examples

195/65R15 to 205/55R16: the diameter goes from 634.5 mm to 631.9 mm (−2.6 mm, −0.41%). With a reading of 100, the true speed is 99.6, so the speedometer reads slightly high. Revolutions per km go from 501.7 to 503.7.

A big change, 265/70R16 to 285/75R16: 777.4 mm to 833.9 mm (+56.5 mm, +7.27%). With the speedometer at 60, the true speed is 64.4 (+4.4), and the wheel turns 381.7 times per km instead of 409.5.

Raising the rim by an inch while keeping the aspect ratio, 205/55R16 to 205/55R17, adds 25.4 mm (+4.02%), because each inch of rim adds 25.4 mm of diameter. At a reading of 100 the true speed is 104.0. A low-profile swap, 205/55R16 to 225/40R18, adds only 5.3 mm (+0.84%) and cuts the sidewall from 112.8 mm to 90.0 mm.

What the tool accepts

• Spaces and a Z before the R are allowed: 205 / 55 ZR 16 is read as 205/55R16. A dash can replace the R, as in 205/55-17.
• Enter only the size. A full marking with a load index and speed rating, such as 205/55R16 91V, is not recognized and shows the size-format message.
• Width must be 125 to 445 mm, the aspect ratio 20 to 95 and the rim 10 to 30 inches. A 100 mm width, for example, is rejected.
• Two identical sizes give a difference of 0 and the note "Same as before".
• A blank speedometer box counts as 100. A speed of 0 or less shows "Please enter a speedometer reading above zero." A missing size shows "Please enter both tyre sizes, for example 205/55R16."