Look at the sidewall of almost any modern sedan, crossover, or sports coupe, and you'll find a string like 245/45R18 stamped into the rubber. Most drivers glance at it, assume it's some kind of serial number, and move on. That string is actually a complete technical spec sheet, compressed into ten characters, telling you exactly how wide the tire is, how tall its sidewall stands, how it's built internally, and what wheel it belongs on. Once you know how to read it, you'll never have to take a tire shop's word for it again.
Let's take that code apart piece by piece, because 245/45R18 is a genuinely useful example—it's one of the most common sizes on performance sedans and mid-size crossovers, so decoding it tells you something you can actually use.
Breaking Down 245/45R18 Number by Number
Start with 245. That's the tire's width in millimeters, measured from one sidewall's outer edge to the other when the tire is mounted and inflated on its specified rim. So 245 means 245 millimeters, or roughly 9.6 inches, across the tread-and-sidewall footprint. Wider numbers mean a wider tire—simple enough—but width alone doesn't tell you much about overall tire height or fit, which is where the next number comes in.
The 45 that follows the slash is the aspect ratio, and this is the number that trips up most non-engineers. It's not a measurement in inches or millimeters—it's a percentage. Specifically, it's the sidewall height expressed as a percentage of the tire's width. A 45 aspect ratio means the sidewall is 45 percent as tall as the tire is wide, so on a 245mm-wide tire, the sidewall works out to about 110 millimeters. Lower aspect ratio numbers mean shorter, stiffer sidewalls, which is the hallmark of a "low-profile" tire. Higher numbers mean taller sidewalls with more cushion built in. This ratio is where sportiness and comfort get traded against each other, and we'll come back to that trade-off shortly.
Next is the R, and this one's simple: it stands for radial, referring to how the tire's internal cords are arranged. Radial construction—where the reinforcing plies run perpendicular to the direction of travel, with a stabilizing belt underneath the tread—has been the standard for essentially all passenger and light-truck tires for decades. You'll occasionally see a D in older or specialty tire literature, indicating diagonal or "bias-ply" construction, but unless you're dealing with a trailer tire, a vintage vehicle, or specialized off-road equipment, R is what you should expect and want.
Finally, the 18 is the wheel diameter in inches—not millimeters, which is a deliberate quirk of tire sizing that mixes metric and imperial units in the same code. That 18 means this tire is built to mount on an 18-inch-diameter wheel. It has nothing to do with the tire's overall height or width; it's purely about matching the tire's inner bead diameter to the wheel it's designed to seat on.
Load Index and Speed Rating: The Codes After the Size
If you look past the 245/45R18 itself, you'll typically find a couple more characters tacked on, something like 98W. These aren't decorative. The number—98 in this example—is the load index, a coded value that corresponds to the maximum weight that single tire can carry when properly inflated. It's not a direct pounds-or-kilograms figure; it's a reference number from a standardized index table, and different numbers on that scale correspond to different maximum load capacities. Passenger car tires commonly fall in a load index range that clusters in the high 80s to low 100s, with larger sedans, crossovers, and anything carrying heavier loads generally sitting toward the higher end of that range. Rather than memorizing exact weight figures, treat the load index as a signal: a higher number means a tire engineered for a heavier maximum load, and you should always match or exceed the load index your vehicle manufacturer specifies, never go below it.
The letter that follows—W in our example—is the speed rating, another coded system indicating the maximum speed the tire is certified to sustain safely under proper load and inflation. Letters like H, V, W, and Y appear frequently on passenger vehicles, with the general pattern being that letters further along in the tire industry's rating scale correspond to higher certified speeds, and performance-oriented vehicles tend to ride on tires with higher speed ratings than economy-focused models. This isn't about how fast you plan to drive; it's about the tire's structural ability to manage heat and maintain integrity at sustained highway and, in some cases, well above highway speeds. As with load index, the safe approach is matching your vehicle's original equipment rating rather than assuming a lower-rated tire will be fine because you don't drive aggressively.
Why You Shouldn't Guess Your Tire Size
Here's the part that matters most practically: you should never eyeball or estimate a compatible tire size based on what looks right or what a previous owner happened to have installed. Every vehicle has a manufacturer-specified tire size, and it's not hidden—it's printed on a sticker inside the driver's door jamb, and it's also listed in the owner's manual. That sticker is the actual authority, not the tire currently mounted on the car, which may already be an aftermarket replacement that doesn't match the original spec. Shops occasionally install a size that's close but not identical, especially on used vehicles, so cross-checking the door jamb sticker against what's actually on the car is a five-minute habit that can save you from compounding somebody else's earlier mistake.
What Happens When You Change the Numbers
Enthusiasts and shops sometimes talk about "plus-sizing"—moving to a larger wheel diameter while adjusting the aspect ratio downward to keep the tire's overall diameter close to stock. This is where understanding the 245/45R18 breakdown actually pays off, because plus-sizing means increasing the 18 while decreasing the 45, aiming to keep the outer diameter roughly constant even as the wheel gets bigger and the sidewall gets shorter. Done correctly, this can sharpen steering response and reduce sidewall flex during hard cornering, which is why it's popular on performance builds. Done carelessly, it throws off your speedometer and odometer accuracy, because both are calibrated around the tire's revolutions per mile at the factory-specified diameter—change that diameter meaningfully and your speedometer will read slightly off, even if the car still drives fine. Shorter, stiffer sidewalls from a lower aspect ratio also transmit more road imperfections into the cabin, trading ride comfort for handling precision, and they leave less cushion to protect the wheel itself from pothole and curb damage.
The AWD Trap: Why Matching Sizes Across Axles Matters
One consequence that catches people off guard involves all-wheel-drive vehicles specifically. On an AWD system, the drivetrain expects all four tires to be rotating at very close to the same rate for a given road speed, since that's how the center differential or transfer case coordinates power delivery front to rear. If you replace two tires with a slightly different size or even a different tread depth from significant wear—effectively changing the rolling diameter on one axle relative to the other—you introduce a speed differential that the drivetrain has to constantly compensate for. Over time this can overheat and damage the transfer case, center differential, or other AWD hardware, sometimes leading to expensive repairs that had nothing to do with the tires themselves failing. This is exactly why AWD vehicle manufacturers are typically strict about replacing tires in complete sets or, at minimum, keeping tread depth closely matched across all four corners, and why a tire shop worth trusting will flag this before selling you just two replacements.
Reading 245/45R18 isn't a party trick—it's the difference between buying a tire that fits your car's actual engineering and buying one that merely looks similar to what was there before.
Key Takeaways
- In 245/45R18, 245 is the tire width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of that width), R denotes radial construction, and 18 is the wheel diameter in inches.
- Load index and speed rating codes that follow the size, such as 98W, indicate maximum load capacity and maximum sustained speed capability, and should meet or exceed your vehicle's original specification.
- Always verify your correct tire size against the door-jamb sticker or owner's manual rather than assuming the currently mounted tires are correct.
- Plus-sizing (larger wheel, lower aspect ratio) can sharpen handling but generally stiffens the ride and can throw off speedometer accuracy if overall diameter isn't preserved.
- On all-wheel-drive vehicles, mismatched tire sizes or uneven tread wear across axles can strain and damage drivetrain components like the transfer case or center differential.
- Bottom line: treat the tire size code as a precise spec, not a suggestion, and when in doubt, match the manufacturer's listed size exactly rather than approximating.
