How to Read a Tire Size: Decoding Those Cryptic Numbers on Your Sidewalls

I'll never forget the first time I stood in a tire shop, staring at a wall of black rubber donuts, completely baffled by the hieroglyphics stamped on their sides. P225/65R17 95H? Might as well have been ancient Sumerian. The salesman rattled off numbers like an auctioneer, and I just nodded along, pretending I understood why one set of numbers cost $400 more than another.

That was fifteen years ago. Since then, I've changed more tires than I care to count, helped countless friends decode their sidewalls, and learned that those numbers aren't just random – they're telling you a story about exactly what your car needs to roll safely down the road.

The Basic Code Structure

Every tire wears its vital statistics on its sidewall, typically in a format that looks something like this: P215/60R16 94T. At first glance, it's intimidating. But once you understand the pattern, it becomes as readable as a nutrition label.

The beauty of this system is its universality. Whether you're buying tires in Detroit or Dubai, these numbers mean the same thing. It's one of those rare instances where the automotive industry actually agreed on something globally – well, mostly. There are still some quirks we'll get into.

Let me break down what each element means, starting from the beginning of that alphanumeric soup.

That First Letter (Or Lack Thereof)

The very first character tells you what kind of vehicle the tire was designed for. Most commonly, you'll see:

P stands for P-metric, meaning passenger vehicle. This covers most cars, SUVs, and light trucks that regular folks drive daily. If you're reading this article, there's a good chance your tires start with P.

LT means Light Truck. These tires are built beefier, designed to handle heavier loads and tougher conditions. You'll find these on pickup trucks, large SUVs, and vans that actually haul things.

Sometimes there's no letter at all. These are Euro-metric tires, which despite the name, are perfectly fine for American vehicles. The sizing is slightly different, but functionally they're interchangeable with P-metric in most cases. I've run both on the same car without issue.

T stands for Temporary – as in spare tire. If you see this, it better be in your trunk, not on your wheels.

The Width Number

After that initial letter comes a three-digit number – this is your tire's width in millimeters, measured from sidewall to sidewall when properly inflated and mounted. So a 225 means the tire is 225 millimeters wide, roughly 8.9 inches.

Here's something that took me years to fully appreciate: wider isn't always better. Sure, wide tires look aggressive and can provide more grip in dry conditions. But they also hydroplane easier, cost more, reduce fuel economy, and can make your car feel sluggish. I learned this the hard way when I put 245s on a car designed for 205s – it handled like a shopping cart in the rain.

The width needs to match your wheel width too. You can't just slap a 285mm tire on a narrow rim and call it good. There's a range of acceptable widths for each wheel, and going outside that range is asking for trouble.

The Aspect Ratio

Right after the width, you'll see a forward slash followed by a two-digit number. This is the aspect ratio – the height of the sidewall expressed as a percentage of the width. A 60 means the sidewall height is 60% of the tire's width.

This number dramatically affects how your car rides and handles. Lower numbers mean shorter sidewalls, which typically translates to better handling but a harsher ride. Higher numbers give you more cushion but can make the car feel less responsive.

I once had a sports car with 35-series tires – they looked fantastic and the car cornered like it was on rails, but hitting a pothole felt like getting punched in the kidneys. When I switched to 45-series tires, I lost a bit of that razor-sharp handling but gained the ability to drive over railroad tracks without checking my dental work afterward.

The Construction Type

After the aspect ratio comes a letter that indicates how the tire is constructed. You'll almost always see R for radial construction. Radials have been the standard since the 1970s because they last longer, provide better fuel economy, and handle better than the bias-ply tires they replaced.

Occasionally you might see D for diagonal or bias-ply construction, but only on trailer tires or vintage car reproductions. If someone tries to sell you bias-ply tires for your daily driver, run away.

There's also B for belted bias, but these are essentially extinct in the passenger car world. I've only seen them on some seriously old-school muscle car restorations where authenticity matters more than performance.

The Wheel Diameter

Next comes another two-digit number – this is your wheel diameter in inches. Yes, we just switched from metric to imperial measurements. Don't ask me why the tire industry decided to mix measurement systems; it's been this way forever and nobody seems interested in fixing it.

This number needs to match your wheel exactly. A 17-inch tire only fits a 17-inch wheel. Period. No fudging, no "close enough." I've seen people try to stretch a 16-inch tire onto a 17-inch wheel. It doesn't end well.

Load Index and Speed Rating

After the size numbers, you'll typically see a two or three-digit number followed by a letter. The number is the load index – how much weight the tire can support. The letter is the speed rating – the maximum speed the tire can safely maintain.

Load indexes run from 0 to 150, though passenger car tires typically fall between 75 and 105. Each number corresponds to a specific weight capacity. A 94 load index means the tire can support 1,477 pounds. Multiply by four tires, and you get your vehicle's maximum load capacity – assuming your tires are the limiting factor, which they usually are.

Speed ratings run from A to Z, though not alphabetically. The system is quirky – H comes after U but before V, for historical reasons that made sense to someone in the 1960s. Most passenger cars use T (118 mph), H (130 mph), or V (149 mph) rated tires. Unless you're planning to explore your car's top speed on the Autobahn, anything H-rated or above is more than sufficient.

Additional Markings That Matter

Beyond the main size designation, tires carry other important information. The DOT code tells you when and where the tire was made. The last four digits are the week and year of manufacture. A tire marked 2419 was made in the 24th week of 2019.

This matters because tires age, even sitting in a warehouse. The rubber compounds break down over time, becoming harder and less grippy. I won't mount tires more than six years old, no matter how good the tread looks. I've seen too many "new" old tires develop sidewall cracks or simply not grip properly.

You'll also find treadwear, traction, and temperature ratings – the UTQG ratings. Treadwear is a comparative number; a 400 treadwear tire should last twice as long as a 200. Traction grades (AA, A, B, or C) indicate wet stopping ability. Temperature grades (A, B, or C) show the tire's ability to dissipate heat.

Special Designations

Some tires carry additional markings for specific capabilities:

M+S or M/S stands for Mud and Snow, indicating the tire meets certain requirements for those conditions. Don't confuse this with the three-peak mountain snowflake symbol, which indicates true winter tire performance. M+S tires are all-seasons that can handle light snow; mountain snowflake tires are proper winter tires.

XL or Extra Load means the tire can handle higher inflation pressures and heavier loads than standard tires of the same size. These are often required on heavier vehicles or European cars.

Run-flat tires have various markings depending on the manufacturer – RFT, ROF, EMT, ZP, or SSR. These can be driven on for a limited distance after losing air pressure. They're convenient but ride harsher and cost significantly more to replace.

Practical Application

Understanding tire sizes becomes really useful when you want to change your tire dimensions. Maybe you want a more aggressive look, better performance, or just found a great deal on a slightly different size.

The key is maintaining the overall diameter. If you go wider, you typically need to decrease the aspect ratio to keep the same overall height. There are online calculators for this, but I'll share the quick mental math: every 10mm wider you go, drop the aspect ratio by about 5 to maintain diameter.

For example, if your car came with 205/55R16 tires and you want to go to 225mm wide, you'd want 225/45R16 to keep roughly the same diameter. This maintains your speedometer accuracy and prevents clearance issues.

Common Misconceptions

Let me address some persistent myths about tire sizing:

"Bigger is always better" – Nope. The best tire size is what your vehicle was engineered for, plus or minus one size in width. Going too far outside the design parameters affects everything from handling to transmission life.

"All tires of the same size are interchangeable" – Not quite. A 225/45R17 91H and a 225/45R17 94V might fit the same wheel, but the load and speed ratings matter. Always meet or exceed your vehicle's requirements.

"You can mix tire sizes for better performance" – Please don't. Running different sized tires can confuse stability control systems, cause handling issues, and wear out differentials. The only exception is vehicles designed for staggered setups, like some sports cars and muscle cars.

The Bottom Line

Those numbers on your tire sidewall aren't just technical gibberish – they're crucial information about your vehicle's only contact with the road. Understanding them helps you make informed decisions about replacements, upgrades, and maintenance.

Next time you're tire shopping, you won't need to rely solely on the salesperson's recommendations. You'll know exactly what those numbers mean and whether that "great deal" on slightly different-sized tires is actually a good idea for your car.

Remember, tires are the single most important safety component on your vehicle. They affect braking, handling, comfort, and fuel economy. Spending a few minutes understanding their language is time well invested. Your car – and your spine on rough roads – will thank you.

Authoritative Sources:

National Highway Traffic Safety Administration. Tire Safety: Everything Rides on It. U.S. Department of Transportation, 2017.

Tire and Rubber Association of Canada. Understanding Tire Markings and Designations. TRAC Technical Bulletin, 2020.

Rubber Manufacturers Association. Tire Information Service Bulletin: Tire Size Designation Systems. RMA Publications, 2018.

Society of Automotive Engineers. SAE J1088: Tire Load Ratings and Inflation Pressures - Trucks, Buses, and Multipurpose Passenger Vehicles. SAE International, 2019.

European Tyre and Rim Technical Organisation. Standards Manual 2021. ETRTO Publications, 2021.