Tire size uses width, sidewall height, and wheel diameter; a code like 225/45R17 shows all three at a glance.
If you’ve ever stared at a tire sidewall and felt like it was written in code, you’re not alone. The good news is that tire size follows a clear pattern. Once you know what each number and letter stands for, you can tell how wide the tire is, how tall its sidewall is, and what wheel it fits.
That matters when you’re buying replacements, checking fitment, or trying to tell whether a different size will change ride height. It also stops a common mistake: matching only the last number and missing the rest of the size code.
How Is Tire Size Measured? Start With The Sidewall
The standard passenger-tire code is printed right on the sidewall. A size such as 225/45R17 is not random. It’s a compact label that gives the tire’s basic dimensions in a mix of millimeters, percentage, and inches.
Take 225/45R17. The first number is the tire’s section width in millimeters. The second number is the aspect ratio, which tells you the sidewall height as a percentage of the width. The final number is the wheel diameter in inches. Michelin lays out that structure clearly in its tire marking explainer.
What 225/45R17 Means
Here’s the code broken into plain English:
- 225 = tire width, measured in millimeters
- 45 = sidewall height, shown as 45% of the width
- R = radial construction
- 17 = wheel diameter, measured in inches
You may also see extra characters before or after that size. A P can mean passenger tire. LT points to a light-truck tire. Numbers and letters after the size, such as 94V, show load index and speed rating. Those matter when replacing a tire, but they are not the size itself.
Width, Sidewall Height, And Wheel Diameter
Each part of the code tells a different part of the tire’s shape. Put together, they give you the tire’s usable dimensions.
Width Is The First Number
The width is measured across the tire from sidewall to sidewall, not across the tread blocks. So a 225 tire is about 225 millimeters wide at its widest point under standard measuring conditions.
There’s one catch. This width is nominal, which means it can shift a bit once the tire is mounted on a wheel. A wider rim can stretch the tire slightly. A narrower rim can pull it inward. That’s why two tires with the same labeled size can measure a little differently in the real world.
Aspect Ratio Is A Percentage
The second number is easy to miss, but it changes how a tire looks and feels. A 45 aspect ratio means the sidewall height is 45% of the tire’s width. On a 225 tire, that works out to 101.25 millimeters.
Lower aspect ratios mean a shorter sidewall. That usually gives the tire a squatter look. Higher aspect ratios mean a taller sidewall, which often brings a softer ride and more sidewall flex.
Wheel Diameter Uses Inches
The last number is the wheel size the tire fits. A tire marked 17 fits a 17-inch wheel. This number is not the tire’s full outside diameter. It is only the diameter of the wheel at the bead seat.
That’s why a 17-inch tire can still look much taller than another 17-inch tire. The wheel may be the same size, but the sidewall height can change the total tire height by a lot.
| Marking | What It Means | What You Learn From It |
|---|---|---|
| P | Passenger-tire prefix | Built for passenger-car use |
| LT | Light-truck prefix | Built for truck or heavy-duty use |
| 225 | Section width in millimeters | How wide the tire is sidewall to sidewall |
| 45 | Aspect ratio | Sidewall height equals 45% of the width |
| R | Radial construction | How the tire is built internally |
| 17 | Wheel diameter in inches | What rim size the tire fits |
| 94V | Load index and speed rating | How much weight and speed the tire is rated for |
If you want the correct replacement size for your car, don’t rely only on the number already on the tire. Use the vehicle placard on the driver’s door jamb or the owner’s manual. NHTSA points drivers to the vehicle’s listed tire specs on its tire safety page, and that’s the safer place to settle any mismatch.
How To Measure Tire Size By Hand
The sidewall code is the easiest route, but you can also measure a tire yourself. This comes in handy with worn sidewalls, older tires, or off-road tires that use a different labeling style.
- Measure the tire’s width at its widest point, sidewall to sidewall.
- Measure the wheel diameter straight across the wheel, not the full tire.
- Measure one sidewall from the rim edge to the tread top.
- Add two sidewalls to the wheel diameter to get full tire diameter.
Using 225/45R17, the sidewall height is 225 × 0.45 = 101.25 mm. Convert that to inches if you want, or stay in metric. Full tire diameter is two sidewalls plus the 17-inch wheel diameter. That lands just under 25 inches.
This kind of math is handy when you’re comparing two tire sizes. It shows whether the new tire will sit taller, leave less fender clearance, or throw off your speedometer reading.
Metric Codes And Flotation Sizes
Most cars, crossovers, and many SUVs use the metric style. Some trucks and off-road tires use flotation sizing instead. That style starts with the tire’s full height, then width, then wheel diameter.
A flotation size such as 31×10.50R15LT reads like this: the tire is about 31 inches tall, about 10.5 inches wide, radial, and built for a 15-inch wheel. Same idea, different order.
| Size Style | Sample Code | Reading Order |
|---|---|---|
| Metric | 225/65R17 | Width, sidewall ratio, construction, wheel diameter |
| Flotation | 31×10.50R15LT | Overall height, width, construction, wheel diameter |
| Euro-Metric | 205/55R16 | Same as metric, often with no P prefix |
| Light Truck Metric | LT265/70R17 | Truck prefix plus metric dimensions |
What Trips People Up
Tire sizing gets messy when people treat one number as the whole story. A few easy mix-ups cause most buying errors.
- Tread width and section width aren’t the same. The labeled width is sidewall to sidewall.
- The last number is not full tire height. It’s only the wheel diameter.
- Same wheel size does not mean same tire height. A 225/45R17 and 225/65R17 fit the same rim diameter but stand at different heights.
- One size can vary by brand. The printed size is nominal. Actual specs can move a little.
- Load and speed ratings still matter. Matching size alone is not enough.
That last point gets skipped a lot. If your car calls for a certain load index or speed rating, your replacement tire should meet or exceed it. The wrong rating can change the way the car carries weight, brakes, and rides.
Picking A Replacement Tire Without Guesswork
When you’re ready to buy, match more than just the main size code. Use this short checklist:
- Start with the driver-door placard or owner’s manual
- Match the full size, not only the wheel diameter
- Check load index and speed rating
- Make sure the tire type fits the vehicle’s use
- Compare overall diameter if you’re switching sizes
Once you read the code with confidence, the whole topic gets a lot less murky. Tire size is measured through a simple format: width in millimeters, sidewall height as a percentage, and wheel diameter in inches. Read those three parts first, then use the rest of the sidewall to confirm fit, load, and speed.
References & Sources
- Michelin.“How to Read Tire Markings and Sidewall Codes.”Shows how tire width, aspect ratio, construction, and wheel diameter are printed on the sidewall.
- National Highway Traffic Safety Administration (NHTSA).“Tire Safety Ratings and Awareness | TireWise.”Directs drivers to the vehicle placard and explains sidewall tire information used when choosing replacements.