MiniCalci

Calculators & Converters
Automotive

Tire Size Calculator

Compare two tire sizes to see the difference in overall diameter and the resulting speedometer impact. Enter width, aspect ratio, and rim size.

Current Tire

New / Comparison Tire

What Does This Tire Size Calculator Do?

This calculator compares two tire sizes (for example, your current tires vs. a size you're considering) and shows the difference in overall diameter along with the resulting speedometer impact — useful when shopping for replacement tires or a different wheel/tire combination.

The Tire Diameter Formula

Sidewall Height (in) = [Width (mm) x (Aspect Ratio / 100)] / 25.4
Overall Diameter (in) = Rim Diameter (in) + (2 x Sidewall Height)

Worked Example: 225/50R17

  • Sidewall height = (225 x 0.50) / 25.4 = 112.5 / 25.4 = 4.43 in
  • Overall diameter = 17 + (2 x 4.43) = 17 + 8.86 = 25.86 in

Speedometer Impact

Since a larger-diameter tire travels farther per revolution, a speedometer calibrated for a smaller tire will read slower than your true speed once the larger tire is installed (and vice versa for a smaller tire). This calculator shows the percentage difference in diameter as an approximation of that effect.

Check your tire wear with the tire life calculator, or browse more automotive tools in the calculator directory.

Frequently Asked Questions

How do I read a tire size like 225/50R17?
225 is the tire's width in millimeters, 50 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel rim diameter in inches.
How is tire diameter calculated?
Sidewall height (mm) = Width x (Aspect Ratio / 100). Convert to inches by dividing by 25.4, then Overall Diameter = Rim Diameter + (2 x Sidewall Height in inches), since the sidewall appears on both the top and bottom of the wheel.
Why does changing tire size affect my speedometer?
Speedometers are calibrated to your original tire's diameter and its resulting wheel revolutions per mile. A larger tire covers more distance per revolution, causing your speedometer to under-read your actual speed. A smaller tire causes it to over-read.
How much of a size difference is safe for a tire swap?
A common rule of thumb is to keep the overall diameter within about 3% of the original tire size to avoid meaningfully affecting speedometer accuracy, ABS/traction control calibration, and gearing.
What is 'plus sizing' in tires?
Plus sizing means moving to a larger rim diameter with a lower-profile (smaller aspect ratio) tire while keeping the overall diameter roughly the same, which is a common way to upgrade wheel appearance without significantly affecting the vehicle's calibrated systems.
Does this calculator account for load rating or speed rating?
No, this tool only compares the physical diameter and resulting speedometer effect based on width, aspect ratio, and rim size. Always check load index and speed rating separately to ensure a replacement tire is appropriate for your vehicle.

Related Calculators

AUTO

Tire Life Calculator

Estimate remaining tire life based on tread wear

Open calculator
CONV

Gas Mileage Calculator

Calculate MPG and trip fuel cost

Open calculator
CONV

Length Converter

Convert between inches, feet, yards, miles, and metric lengths

Open calculator
How this is calculated: This tool uses standard, well-established formulas for automotive calculations. Results are estimates for informational and planning purposes — always double-check figures for anything with real-world consequences (e.g. ordering materials or submitting grades).