Tire Size Calculator
Decode any tire size code, calculate actual tire dimensions, compare two tire sizes, and check the effect on your speedometer accuracy.
Tire sizes are formatted as: 225/55R17 — Width (mm) / Aspect Ratio / Rim Diameter (inches)
Tire 1 (Original / Stock)
Tire 2 (Optional — for comparison)
Tire 1 Dimensions
Tire 2 Dimensions
Comparison (Tire 2 vs Tire 1)
How to Read a Tire Size
Every tire has a standardized size code embossed on the sidewall. For example: 225/55R17
| Part | Value | Meaning |
|---|---|---|
| Width | 225 | Section width in millimeters |
| Aspect Ratio | 55 | Sidewall height as % of width |
| Construction | R | Radial (almost all modern tires) |
| Rim Diameter | 17 | Wheel diameter in inches |
Sidewall height = Width × (Aspect Ratio ÷ 100). For 225/55: 225 × 0.55 = 123.75mm = 4.87 inches.
Overall diameter = Rim diameter + (2 × sidewall height in inches)
Speedometer Impact of Different Tire Sizes
Your speedometer is calibrated for your factory tire's rolling circumference. A larger tire has a greater circumference and travels farther with each rotation — so your speedometer will under-read your actual speed. A smaller tire will cause it to over-read.
A difference of more than ±3% from stock is generally considered significant and may affect odometer accuracy, ABS calibration, and traction control systems.
Plus Sizing
"Plus sizing" means going to a larger rim diameter (e.g., 17" to 18") while reducing the aspect ratio to keep the overall diameter the same. This gives a wider, lower-profile look and can improve handling, but often at the expense of ride comfort.
Frequently Asked Questions
What does the load index and speed rating mean?
After the size code (e.g., 225/55R17 97V): 97 is the load index (a coded maximum load per tire), and V is the speed rating (maximum sustained speed — V = 149 mph). Always match or exceed the manufacturer's minimum rating.
How much difference in size is acceptable?
Most mechanics recommend keeping overall diameter within ±3% of stock to avoid affecting speedometer accuracy, ABS, and traction control. Rim diameter changes should be offset by aspect ratio changes to maintain overall diameter.
Can I use different size tires front and rear?
Staggered fitments are common on performance cars (wider rear tires). However, all-wheel-drive vehicles generally require all four tires to be the same size to prevent drivetrain damage.
