Tire Size Calculator
Compare two tire sizes to see the diameter and speedometer difference.
Try an example
+1.81%
About this calculator
How to Read a Tire Size
Every car tire carries a code on its sidewall, for example P225/50R17 94V. Each part describes one characteristic of the tire, and with the numbers you can work out its exact dimensions. This tire size calculator decodes a size and compares two tire sizes side by side, so you can see how a different tire changes diameter, width, sidewall height and speedometer accuracy.
What the Code Means
- Intended use (P): P is for passenger cars, SUVs, minivans and light pickups; LT is light truck for heavy loads and towing; ST is special trailer; T is temporary spare.
- Section width (225): the width of the tire in millimeters from sidewall to sidewall.
- Aspect ratio (50): the sidewall height as a percentage of the width — here 50% of 225 mm is 112.5 mm.
- Construction (R): R means radial, the near-universal modern design.
- Wheel diameter (17): the rim size in inches that the tire fits.
- Load index (94) and speed rating (V): the maximum load per tire and the top sustained speed the tire is rated for.
Worked Example
Going from a 225/50R17 to a 235/45R18 increases the overall tire diameter by about 1.81% — a small enough change that most speedometers won't need recalibration.
Calculating Overall Diameter
Overall diameter = Wheel diameter + 2 × Sidewall height. For 225/50R17: sidewall = 225 × 0.50 = 112.5 mm ≈ 4.43 in; diameter = 17 + 2 × 4.43 ≈ 25.9 in. The diameter determines how far the car travels in one turn of the wheel, and so how accurate your speedometer and odometer are.
Why Compare Tire Sizes?
- Speedometer accuracy: a larger overall diameter means the car travels farther per revolution, so the speedometer reads lower than your real speed. Keeping the diameter within about 3% of the original is a common guideline.
- Fit and clearance: wider or taller tires can rub against the fender or suspension.
- Handling and comfort: lower-profile tires with shorter sidewalls feel sharper but ride harder.
- Fuel economy, gearing and braking shift slightly with size and weight.
"Plus-sizing" means fitting a larger wheel with a lower-profile tire so the overall diameter stays about the same. For rolling distances and wheel math, use the Circle Calculator, check speeds with the Speed Calculator, and verify pressure units with the Pressure Calculator.
Worked example
Going from a 225/50R17 to a 235/45R18 increases the overall tire diameter by about 1.81% — a small enough difference that most speedometers won't need recalibration.
Frequently asked questions
Why does tire size affect my speedometer?
Your speedometer calculates speed from wheel rotation speed, calibrated to your original tire's diameter. A larger-diameter tire covers more ground per rotation, so the speedometer under-reads your actual speed; a smaller tire makes it over-read.
How much of a diameter difference is considered safe?
A common rule of thumb is staying within about 3% of the original diameter to avoid meaningfully affecting speedometer accuracy, gearing, and ABS/traction control systems — though always check your vehicle manufacturer's specific tolerances.
