How do I read a tire size like P225/65R17?

P = passenger car; 225 = section width in mm; 65 = aspect ratio (sidewall height is 65% of 225 mm = 146.25 mm); R = radial construction; 17 = rim diameter in inches.

How is tire overall diameter calculated?

Overall diameter = rim diameter × 25.4 mm/in + 2 × sidewall height. For P225/65R17: (17 × 25.4) + (2 × 146.25) = 431.8 + 292.5 = 724.3 mm (28.52 in).

Why does tire diameter matter for speedometer accuracy?

Your speedometer is calibrated for a specific tire diameter. Fitting a larger or smaller diameter changes the distance traveled per wheel revolution, making the speedometer read low or high. A difference of more than 3% is generally noticeable.

What is the difference between section width and tread width?

Section width (the first number in the tire code) is the widest point of the unmounted tire, measured from sidewall to sidewall. Tread width is the portion of the tire that actually contacts the road and is narrower than the section width.

What does the aspect ratio represent?

The aspect ratio is the sidewall height expressed as a percentage of the section width. A 65-series tire has a sidewall height equal to 65% of its width. Lower aspect ratios (e.g. 35 or 45) mean shorter sidewalls and a sportier, lower-profile look.

Tire Size Calculator — Sidewall, Diameter, and Circumference

Tire Size Calculator

A tire code like P225/65R17 encodes the three dimensions you need to calculate sidewall height, overall diameter, and circumference. Understanding these numbers helps when comparing tire sizes, checking speedometer accuracy after a size change, or computing rolling circumference for odometer calibration.

The formula

section_width_mm  = 225        (the first number)
aspect_ratio      = 65         (the second number, as a percentage)
rim_diameter_in   = 17         (the last number, in inches)

sidewall_mm   = section_width_mm × (aspect_ratio / 100)
              = 225 × 0.65 = 146.25 mm

overall_diam_mm = rim_diameter_in × 25.4 + 2 × sidewall_mm
                = 17 × 25.4 + 2 × 146.25
                = 431.8 + 292.5 = 724.3 mm

circumference_mm = π × overall_diam_mm
                 = 3.14159 × 724.3 = 2275 mm

Practical examples

Example 1 — P225/65R17 (standard SUV tire).

Sidewall: 225 × 0.65 = 146.25 mm (5.76 in)
Overall diameter: 431.8 + 292.5 = 724.3 mm (28.52 in)
Circumference: π × 724.3 = 2,275 mm (89.57 in)

Example 2 — 205/55R16 (common compact car tire).

Sidewall: 205 × 0.55 = 112.75 mm (4.44 in)
Overall diameter: 16 × 25.4 + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm (24.88 in)

Example 3 — Checking speedometer accuracy after upsizing. Original P215/60R16; new P225/65R17. Original diameter 663.4 mm; new diameter 724.3 mm. The new tire is ~9.2% larger in diameter, so the speedometer would read ~9.2% low at any given speed. A recalibration is advisable.

Common mistakes

Using the new tire's section width instead of the original. When comparing tire sizes, always use the same reference tire as your baseline.
Forgetting that two sidewalls add to the diameter. The formula adds 2 × sidewall height (one for each side), not 1 × sidewall.
Assuming all tires of the same code are the same size. Actual dimensions vary by manufacturer — this calculator gives nominal dimensions. Always check the tire manufacturer's published measurements for precision work.

International and regional variations

Prefix	Meaning	Examples
P	Passenger car (US P-metric)	P225/65R17
LT	Light truck	LT265/70R17
ST	Special trailer	ST205/75R14
(none)	Euro-metric (same math, no prefix)	225/65R17
C	Commercial van (European)	195/70R15C

Quick reference — diameter by common tire code

Tire code	Sidewall (mm)	Overall diameter (mm)	Overall diameter (in)
P195/65R15	126.75	634.9	24.99
P205/55R16	112.75	631.9	24.88
P215/60R16	129.0	664.4	26.16
P225/65R17	146.25	724.3	28.52
P265/70R17	185.5	803.8	31.65

For a full decoder of all tire code segments — including speed rating letters and load index values — see the Tire Size Chart: Speed Ratings & Load Index →.

Section width (mm)

Aspect ratio (%)

Rim diameter (in)

Sidewall height146.3 mm / 5.76 in

Overall diameter724.3 mm / 28.52 in

Overall circumference2,275.5 mm / 89.58 in

Calculated from the nominal tire code. Actual dimensions vary by manufacturer and load.

Frequently Asked Questions

How do I read a tire size like P225/65R17?: P = passenger car; 225 = section width in mm; 65 = aspect ratio (sidewall height is 65% of 225 mm = 146.25 mm); R = radial construction; 17 = rim diameter in inches.
How is tire overall diameter calculated?: Overall diameter = rim diameter × 25.4 mm/in + 2 × sidewall height. For P225/65R17: (17 × 25.4) + (2 × 146.25) = 431.8 + 292.5 = 724.3 mm (28.52 in).
Why does tire diameter matter for speedometer accuracy?: Your speedometer is calibrated for a specific tire diameter. Fitting a larger or smaller diameter changes the distance traveled per wheel revolution, making the speedometer read low or high. A difference of more than 3% is generally noticeable.
What is the difference between section width and tread width?: Section width (the first number in the tire code) is the widest point of the unmounted tire, measured from sidewall to sidewall. Tread width is the portion of the tire that actually contacts the road and is narrower than the section width.
What does the aspect ratio represent?: The aspect ratio is the sidewall height expressed as a percentage of the section width. A 65-series tire has a sidewall height equal to 65% of its width. Lower aspect ratios (e.g. 35 or 45) mean shorter sidewalls and a sportier, lower-profile look.