How to Read a Tire Size

A tire size like 225/45R17 91V looks like a code, but it's really just six pieces of information stacked together. Here's what each part means.

Breaking down 225/45R17 91V

225 / 45 R 17 91 V

225: section width in millimeters (the tire's width, sidewall to sidewall)
45: aspect ratio / profile, the sidewall height as a % of section width
R: construction type, radial (almost every tire made today)
17: rim diameter in inches (the wheel size the tire is built for)
91: load index (the maximum weight the tire can carry)
V: speed rating (the maximum sustained speed the tire is rated for)

Section width

The first number, 225 in this example, is the tire's section width in millimeters, measured from the widest point of one sidewall to the other when mounted on its design rim. It is not the same as tread width, which is usually a little narrower.

Aspect ratio (profile)

The number after the slash, 45, is the aspect ratio: the sidewall height as a percentage of the section width. A 45-profile tire has a sidewall that's 45% as tall as it is wide:

Sidewall height = section width × (aspect ratio ÷ 100)

For 225/45: 225 × 0.45 = 101.25 mm. Lower numbers mean a shorter, stiffer sidewall (a sharper look, harsher ride); higher numbers mean a taller sidewall (more comfort, more sidewall flex).

Construction: R (radial)

The letter before the rim size almost always reads R, for radial construction. The internal ply cords run radially across the tire. Radials have been the standard for passenger cars for decades; you'll rarely encounter the older "B" (bias-belted) or "D" (diagonal/bias-ply) markings outside of vintage or specialty tires.

Rim diameter

The number right after "R", 17, is the wheel (rim) diameter the tire is designed to mount on, in inches. This has to match your wheel exactly; unlike width and profile, rim diameter isn't something you can mix and match.

Load index and speed rating

The trailing digits and letter, 91V, aren't part of the size itself, but they matter for safety and legality:

Load index (91) is a code for the maximum load the tire can carry at full inflation. It maps to a kilogram figure via a standard table (91 = 615 kg per tire). Never fit a tire with a lower load index than your car's original spec.
Speed rating (V) is a letter for the tire's maximum sustained speed. Common ratings run roughly H (210 km/h), V (240 km/h), W (270 km/h), Y (300 km/h). As with load index, don't go below your car's original rating.

Why this matters for fitment

Section width and aspect ratio are exactly what feed into rolling diameter, poke and speedometer accuracy when you change tire size. If you're comparing a current size against a new one, you need the width and profile from both sizes:

Rolling diameter = (rim diameter in inches × 25.4) + 2 × (section width × aspect ratio ÷ 100)

Load index and speed rating aren't cosmetic. Fitting a tire rated below your car's original spec can be unsafe and may violate local regulations. Check your door-jamb placard or owner's manual for the minimum rating.

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