A tire size like 225/45R17 91V packs in the tire's width, sidewall height, construction, wheel size, load capacity and speed rating. This calculator decodes it and compares two sizes side by side, including how a change would affect your speedometer.
How to read a tire size
Take 225/45R17 91V as the example:
| Part | Meaning |
|---|---|
225 | Section width in millimetres, sidewall to sidewall. |
45 | Aspect ratio: the sidewall is 45% as tall as the tire is wide (about 101 mm). |
R | Radial construction (nearly all modern tires). |
17 | Fits a 17-inch wheel. |
91 | Load index: up to 615 kg (about 1,356 lb) per tire. |
V | Speed rating: up to 149 mph sustained. |
225- Meaning
- Section width in millimetres, sidewall to sidewall.
45- Meaning
- Aspect ratio: the sidewall is 45% as tall as the tire is wide (about 101 mm).
R- Meaning
- Radial construction (nearly all modern tires).
17- Meaning
- Fits a 17-inch wheel.
91- Meaning
- Load index: up to 615 kg (about 1,356 lb) per tire.
V- Meaning
- Speed rating: up to 149 mph sustained.
A leading P means a P-metric passenger tire and LT a light-truck tire. The size is on the tire's sidewall; the size your vehicle came with, with the recommended pressure, is on the placard in the driver's door jamb.
How the comparison is calculated
- Sidewall height = width × aspect ratio ÷ 100.
- Overall diameter = wheel diameter + 2 × sidewall height.
- Circumference = π × diameter.
- Revolutions per mile = 63,360 inches ÷ circumference.
- Actual speed = indicated speed × new diameter ÷ original diameter.
A taller tire covers more ground per revolution, so the speedometer and odometer read low: at an indicated 60 mph you're going a little faster. A shorter tire makes them read high.
Changing tire size safely
Many tire shops aim to keep a replacement within about 3% of the original overall diameter. Beyond the math, check:
- Load index and speed rating: don't go below what your door placard specifies.
- Clearance: wider or taller tires can rub on suspension or bodywork at full lock or under load.
- Wheel width: each tire size fits a range of wheel widths.
- Electronics: ABS, stability control and some driver-assist systems are calibrated for the original rolling diameter.
- All four corners: on AWD vehicles, mismatched diameters between axles can strain the drivetrain.
When in doubt, choose a size your vehicle maker lists as approved for your model.
Frequently asked questions
What does 225/45R17 mean?
A tire 225 mm wide, with a sidewall 45% of that width, of radial construction, made for a 17-inch wheel.
Can I put a different tire size on my car?
Often, within limits. Stay close to the original overall diameter, keep the load index and speed rating at or above the placard's, and check fitment with a tire shop.
Will bigger tires change my speedometer?
Yes. A larger overall diameter makes the speedometer and odometer read lower than your actual speed and distance. The calculator shows by how much.
Where do I find my tire size?
On the sidewall of the tire, and on the placard on the driver's door jamb, which also lists the recommended pressure. See our guide on checking tire pressure and tread.
Results are estimates from the numbers you enter and, where noted, EPA ratings. Not financial advice. Page updated .