Tire size calculator,
compare sizes & check speedo error.

Field guide

How to read a tire size and why the numbers matter.

Every tire sold in North America carries a standardized size code moulded into its sidewall. Understanding this code is essential when shopping for replacements, upsizing for aesthetics, or downsizing for winter rubber. The metric passenger tire format looks like this:

225 / 45 R 17

• 225: Section width (mm). The width of the tire from sidewall to sidewall at its widest point, in millimetres, when mounted on the recommended rim width and inflated to spec. Wider tires offer more contact patch and cornering grip; narrower tires cut through snow and have less rolling resistance.
• 45: Aspect ratio (%). The sidewall height expressed as a percentage of the section width. A 45-series tire on a 225 mm-wide tire has a sidewall of 225 × 0.45 = 101.25 mm. Lower aspect ratios (30, 35, 40) mean short sidewalls and sharper handling; higher ratios (65, 70, 75) mean taller sidewalls and a softer, more comfortable ride.
• R: Radial construction. Almost all modern tires are radial (the 'R' designation), meaning the internal cord plies run perpendicular to the direction of travel. Diagonal-ply and bias-belted tires (rare today) use 'D' or 'B'.
• 17: Rim (wheel) diameter (inches). The diameter of the wheel the tire is designed to fit, in inches. This must match your wheel exactly. A 17-inch tire cannot mount on an 18-inch rim.

How overall diameter is calculated

The overall tire diameter is what affects your speedometer, odometer, and ground clearance:

1. Sidewall height (mm) = Section width × (Aspect ratio / 100)
   e.g., 225 × 0.45 = 101.25 mm per side
2. Sidewall height (inches) = Sidewall mm / 25.4
   e.g., 101.25 / 25.4 = 3.986"
3. Overall diameter = Rim diameter + 2 × Sidewall height (inches)
   e.g., 17 + 2 × 3.986 = 24.97"
4. Circumference = π × Overall diameter
   e.g., π × 24.97 = 78.45"

Why overall diameter matters more than width

When changing tire sizes, the single most critical number is the overall diameter. Every time your tires rotate once, your car travels exactly one tire circumference. Your vehicle's ABS, traction control, and speedometer all count wheel rotations and assume a specific circumference. If you install tires with a larger overall diameter, your ECU counts fewer revolutions per mile and your speedometer reads lower than your true speed.

A general rule of thumb: keep your new tire's overall diameter within ±3% of the original. Beyond that, speedometer error becomes significant, and on some vehicles, the ABS and stability control may behave incorrectly, potentially triggering warning lights.

Speedometer error explained

Your speedometer is calibrated to your factory tire circumference. The error from a size change is:

Error (%) = (New circumference / Original circumference − 1) × 100

A positive error means the new tire is larger. Your speedometer reads lower than your true speed (you're going faster than it says). A negative error means the new tire is smaller. Your speedometer reads higher than your true speed.

Example: swapping a 225/45R17 for a 235/45R17 increases the circumference by about 1.4%. At a speedo reading of 60 mph, your true speed is 60 × 1.014 = 60.8 mph. Not dramatic, but larger changes can accumulate.

Plus-sizing and staggered fitments

Plus-sizing (also called "plus-one," "plus-two," etc.) means fitting a larger wheel with a lower-profile tire while keeping the overall diameter roughly the same. For example, upgrading from 16-inch to 17-inch wheels typically means going from a 205/55R16 to a 215/50R17 — the diameter difference is under 1%.

The benefits of plus-sizing:

• Wider tires improve dry grip and cornering response.
• Lower profile sidewalls reduce flex and improve steering precision.
• Larger wheels improve brake cooling and often look better.

The drawbacks:

• Lower profile sidewalls transmit more road imperfections into the cabin — the ride becomes firmer and road noise increases.
• Shorter sidewalls are more vulnerable to damage from potholes and curbs.
• Wider tires require more clearance in the wheel arch and may rub on suspension components at full lock.
• Higher-diameter wheels are heavier (increased unsprung mass).

Common plus-size substitutions

Understanding load index and speed rating

The tire size code is often followed by additional characters indicating load capacity and maximum speed:

225/45R17 91W

• 91: Load index. A numeric code indicating the maximum weight the tire can support at full inflation. Load index 91 = 615 kg (1,356 lbs). Your replacement tires must meet or exceed your vehicle's original load index.
• W: Speed rating. The maximum sustained speed the tire is designed for. W = 270 km/h (168 mph). Common ratings: T (190 km/h), H (210 km/h), V (240 km/h), W (270 km/h), Y (300 km/h). Never fit tires with a lower speed rating than the vehicle manufacturer specifies.

Winter tires and staggered fitments

Winter tires are typically the same overall diameter as your summer tires but narrower, e.g., 215/50R17 instead of 245/40R18. The narrower section width creates higher contact-patch pressure that cuts through snow and slush more effectively than a wide tire. This size change has a minimal overall diameter difference but is acceptable because it's seasonal.

On rear-wheel-drive cars with staggered fitments (wider rear tires, e.g. 255/35R19 rear vs. 225/40R19 front), winter tires are often fitted in a non-staggered, square setup (same size front and rear) — allowing you to rotate them and extend their life.