How to read tire sizes
The string on your sidewall — something like 225/45R17 — isn't random. It packs the
width, the sidewall height, and the wheel size into a few characters. Once you can read it, swaps
and upgrades stop being guesswork.
Decoding 225/45R17
| Part | Means |
|---|---|
| 225 | Tread width in millimeters. |
| 45 | Aspect ratio — sidewall height as a percent of width. Here the sidewall is 45% of 225 mm. |
| R | Radial construction (almost all modern tires). |
| 17 | Wheel diameter in inches the tire fits. |
A lower aspect ratio (a "lower profile") means a shorter sidewall — sharper handling, a firmer ride, and less cushion against potholes. A higher number rides softer and is more forgiving on rough pavement.
Calculating the sidewall height
The sidewall height isn't shown directly — you derive it from the width and aspect ratio:
sidewall height (mm) = width (mm) × aspect ratio ÷ 100
For 225/45R17: 225 × 45 ÷ 100 = 101.25 mm. That's the height of
the rubber between the rim lip and the road contact patch, on each side.
Overall diameter is what really matters
Add up the wheel diameter plus two sidewalls and you get the tire's overall diameter — how tall the whole wheel-and-tire assembly is. That number drives how far the car travels per revolution, which is why it affects your speedometer, odometer, and — for vehicles with electronic systems — transmission shift points and traction control calibration.
Plus-sizing is going to a bigger wheel while dropping the aspect ratio so the overall diameter stays about the same — bigger rim, shorter sidewall, same rolling distance. Keep the overall diameter within roughly 3% of stock unless you know what you're changing.
Worked example: plus-sizing from stock
Say your car comes stock on 215/55R16 wheels. You want to upgrade to 17-inch rims. The goal is to keep the overall diameter close to stock so nothing calibrated to the original size is thrown off.
Step 1 — Calculate the stock overall diameter
- Sidewall height:
215 × 55 ÷ 100 = 118.25 mm - Two sidewalls in inches:
(118.25 × 2) ÷ 25.4 = 9.31 inches - Overall diameter:
9.31 + 16 (wheel) = 25.31 inches
Step 2 — Find a 17-inch size that matches
Try 225/45R17:
- Sidewall:
225 × 45 ÷ 100 = 101.25 mm - Two sidewalls in inches:
(101.25 × 2) ÷ 25.4 = 7.97 inches - Overall diameter:
7.97 + 17 = 24.97 inches
The difference from stock is (24.97 − 25.31) ÷ 25.31 = −1.3% —
well within the 3% guideline. Your speedometer will read about 1.3% high (reading a little
fast), which is negligible. Use the Tire Size
Calculator to compare any two sizes and see the exact speedometer offset.
Other markings on the sidewall
The size code is the main string, but most tires have additional markings worth knowing.
| Marking | What it means |
|---|---|
| 91V | Load index (91 = 1,356 lbs max per tire) + speed rating (V = 149 mph max). Both matter for safety. |
| XL or RF | Extra Load (reinforced) — higher load capacity than standard at the same size. Common on European cars. |
| M+S | Mud + Snow rated. Minimum all-season traction — not the same as a dedicated winter tire. |
| Three-peak mountain snowflake | Meets a higher winter traction standard than M+S. Look for this if you drive in snow regularly. |
| DOT + 4-digit code | Manufacturing date: last four digits are week + year. e.g., 2223 = 22nd week of 2023. |
| RSC or MOE | Run-flat tire (manufacturer-specific codes). Can be driven on briefly at reduced speed after a puncture. |
Never replace a run-flat tire with a standard tire unless you also have a spare — run-flats are specifically chosen for vehicles with no spare in the trunk.
Aspect ratio and what it trades off
Aspect ratio is the most misunderstood number in the size code. Higher isn't better or worse — it's a tradeoff.
- Lower aspect ratio (35–45): shorter sidewall, stiffer lateral feel, quicker steering response. Common on performance cars. More vulnerable to pothole damage because there's less rubber to absorb impact.
- Mid range (50–60): the most common range for everyday vehicles — a balance of ride comfort and handling.
- Higher aspect ratio (65–80): taller sidewall, more cushion over rough surfaces. Standard on trucks, SUVs, and economy cars where ride comfort and load capacity matter more than handling precision.
When shopping for replacement tires, changing the aspect ratio changes the overall diameter (unless you adjust width or wheel size to compensate), which triggers all the speedometer and odometer effects described above.
Common mistakes
- Confusing width and aspect ratio. The first number is width in millimeters; the second is aspect ratio as a percentage. A 225/65 tire is narrower than a 245/45 tire — and also has a much taller sidewall. They're completely different shapes.
- Matching only the wheel diameter, not the full size. Any 17-inch tire fits a 17-inch wheel in the sense it'll mount — but if the overall diameter is 5% bigger than stock, it can rub on fenders, throw off the ABS, and violate your odometer readings. Diameter match matters, not just the rim size.
- Ignoring the load index when upgrading. If you switch to a tire with a lower load index than the original, you're under-specifying the tire for your vehicle's weight. Always match or exceed the original load rating.
- Treating M+S as a winter tire. Mud and Snow labeling means the tire passed a minimum threshold — it doesn't mean it's suitable for serious winter driving. If you're in a region with real snow and ice, look for the three-peak mountain snowflake symbol instead.