What Is the 2nd Number in Tire Size? | What It Tells You

The second number on a tire is the aspect ratio, which shows sidewall height as a percentage of the tire’s width.

If the sidewall code on your tire looks like alphabet soup, you’re not alone. A size like 225/55R17 packs a lot into a short string, and the second number is one of the parts that drivers mix up most often. Once you know what it means, tire shopping gets a lot less murky.

The second number in a tire size is the aspect ratio. In plain English, it tells you how tall the tire’s sidewall is compared with the tire’s width. So in 225/55R17, the “55” means the sidewall height is 55% of the tire’s 225 mm width.

That single number shapes more than the tire’s look. It affects ride softness, steering feel, wheel-gap appearance, and how much rubber sits between the rim and the road. It also helps you spot when a replacement tire will change the overall diameter of your setup.

What Is the 2nd Number in Tire Size? It’s The Aspect Ratio

Take a common size such as 205/60R16. The first number, 205, is the tire’s width in millimeters. The second number, 60, is the aspect ratio. The letter after that, usually R, marks radial construction, and the next number, 16, is the wheel diameter in inches.

So the second number is not the tread depth, load rating, or speed rating. It is a percentage. A 60-series tire has a sidewall height that equals 60% of its width. A 45-series tire has a shorter sidewall than a 60-series tire if the width stays in the same ballpark.

How The Math Works

Here’s the easy formula: tire width × aspect ratio = sidewall height. You just convert the ratio into a decimal first. With a 225/55R17 tire, the sidewall height is 225 × 0.55, which comes out to 123.75 mm.

You can use that same math on any passenger tire size. A 225/45R17 tire would have a sidewall height of 101.25 mm. That’s a big drop while the wheel diameter stays the same. That’s why two tires can fit the same rim yet look and feel different on the road.

Second Number In A Tire Size And What It Changes

The aspect ratio changes the tire’s profile. A higher number means a taller sidewall. A lower number means a shorter sidewall. That one shift brings a ripple effect across comfort, handling, and curb protection.

• Higher aspect ratio: more sidewall, more cushion over rough pavement, and a fuller tire shape.
• Lower aspect ratio: less sidewall flex, a firmer feel, and a sharper turn-in feel.
• Taller sidewall: more buffer between the rim and potholes.
• Shorter sidewall: a sportier stance, but less room for hard hits.

That doesn’t mean lower is always better or higher is always safer. It depends on the car, wheel size, suspension tuning, and the roads you drive most often. A family sedan on broken city streets often feels better with more sidewall. A performance coupe may feel tighter and more direct with less.

NHTSA’s tire marking material explains that the two-digit number after width is the aspect ratio, while Bridgestone’s sizing page notes that lower aspect ratios tend to sharpen handling, with a tradeoff in ride comfort. You can read the official wording on NHTSA’s tire safety brochure and Bridgestone’s page on determining tire size.

What A Lower Number Usually Feels Like

Drivers often notice a lower-profile tire in two places right away: over rough pavement and during lane changes. The car may feel more tied down in bends, yet small bumps can feel sharper. There is also less sidewall to absorb pothole blows, so wheel damage can become a bigger risk if you hit a deep hole at speed.

On the flip side, a taller sidewall can smooth out a crusty road and give the tire a rounder, meatier look. That extra rubber can also help shield the rim when you brush a curb. The tradeoff is a softer response when you flick the steering wheel.

How To Read The Full Tire Size At A Glance

Once the second number clicks, the rest of the code starts to make sense too. Here’s a clean breakdown using a common passenger-car size.

Marking	What It Means	Why It Matters
P	Passenger tire category	Shows the tire type the size was written for
225	Width in millimeters	Sets the tire’s section width from sidewall to sidewall
55	Aspect ratio	Shows sidewall height as 55% of the width
R	Radial construction	Marks the tire’s internal build type
17	Wheel diameter in inches	Tells you which rim size the tire fits
94	Load index	Points to how much weight one tire is rated to carry
V	Speed rating	Shows the speed class assigned to the tire
XL	Extra load marking	Shows a higher load capacity than the base version

That table also shows why the second number gets so much attention. It sits near the front of the size code, and it changes the tire’s shape in a way you can see. Width and wheel diameter matter too, yet the aspect ratio often does the most to alter the tire’s profile.

Common Tire Sizes And What The Second Number Means

Here are a few real-world examples. Once you read them side by side, the pattern jumps out right away.

Examples You’ll Spot On Daily Drivers

• 195/65R15: the sidewall height is 65% of 195 mm, so it’s a taller-profile tire.
• 215/55R17: the sidewall height is 55% of 215 mm, giving a middle-ground profile.
• 235/40R18: the sidewall height is 40% of 235 mm, so the sidewall is shorter and stiffer.
• 275/35R19: the sidewall height is 35% of 275 mm, which is low-profile territory.

Here’s the part that trips people up: a wider tire with a lower aspect ratio can still have a sidewall height close to that of a narrower tire with a higher ratio. That’s why you can’t judge the second number by itself. You need the width right next to it.

Take 225/55R17 and 245/50R17. One uses 55, the other uses 50, yet their sidewall heights are close because the second tire is wider. That’s also why tire shops compare full size specs, not one number in isolation.

Tire Size	Sidewall Height	Profile Feel
195/65R15	126.75 mm	Taller and cushier
215/55R17	118.25 mm	Balanced daily-driver feel
225/45R17	101.25 mm	Shorter, firmer profile
275/35R19	96.25 mm	Low-profile, rim sits closer to the road

When The Second Number Matters Most

You’ll care about the aspect ratio most when replacing tires, changing wheel sizes, or trying to keep the speedometer close to stock. Swap to a shorter sidewall without adjusting the width or wheel size and the full tire diameter can shrink. Go the other way and it can grow.

That change can affect wheel-well clearance, ride height, and the way the car reads speed and distance. It can also alter how the car feels under braking and through turns. That’s why sticking close to the factory overall diameter is the usual move unless you’ve planned the size change carefully.

Smart Checks Before You Buy

• Match the new tire to the wheel diameter exactly.
• Check the car maker’s placard for the stock size.
• Compare full dimensions, not just the aspect ratio.
• Make sure the load index and speed rating meet or beat the original spec.

If you’re changing wheel size, the aspect ratio often drops to keep the outer diameter close to the factory setup. That’s why a car might move from 205/60R16 to 225/45R18 during an upsized wheel swap. Bigger rim, shorter sidewall, close overall height.

The Takeaway

The second number in a tire size is the aspect ratio. It tells you the sidewall height as a percentage of the tire’s width, and that single value shapes ride feel, handling feel, and the tire’s visual profile. Read it with the width beside it, and the whole sidewall code starts to click.