Tire Size Calculator
The Tire Size Calculator compares two tire sizes (in the standard width/aspect-ratio/rim format) to show the difference in overall diameter and how it affects speedometer accuracy. The two most common reasons to run this comparison are "plus sizing" — moving to a larger wheel with a lower-profile tire while keeping the overall diameter close to stock — and simply checking fitment before buying a non-standard replacement size. Because anti-lock brakes, traction control, and stability control are all calibrated to the factory tire's rotational speed, a large enough diameter mismatch can throw off those systems in addition to your speedometer and odometer.
How This Tool Works
Enter both tire sizes in width/aspect-ratio/rim format (found on your tire's sidewall, e.g. 225/60R16) to compare their overall diameters.
Formula & Method
Sidewall height (mm) = tire width × (aspect ratio ÷ 100). Overall diameter (in) = (sidewall height × 2 ÷ 25.4) + rim diameter.
Example Calculation
Example: comparing a stock 215/65R16 to an aftermarket 225/60R17 ("plus one" fitment). Original: sidewall height = 215 × (65 ÷ 100) = 139.75 mm; diameter = (139.75 × 2 ÷ 25.4) + 16 ≈ 27.00 in. New: sidewall height = 225 × (60 ÷ 100) = 135 mm; diameter = (135 × 2 ÷ 25.4) + 17 ≈ 27.63 in. The new tire is about 2.32% larger in diameter — within the commonly recommended 3% tolerance — and your speedometer would read 60 mph at a true speed of about 61.4 mph, since the larger tire covers more ground per wheel rotation than the (still stock-calibrated) speedometer accounts for.
Frequently Asked Questions
What do the numbers in a tire size mean?+
In "225/60R16," 225 is the tire width in millimeters, 60 is the aspect ratio (sidewall height as a % of width), and 16 is the rim diameter in inches.
How much diameter difference is safe?+
Most tire professionals recommend staying within about 3% of the original overall diameter to avoid speedometer errors and clearance or handling issues.
Does a different tire size affect my speedometer?+
Yes — since speedometers calculate speed from wheel rotations, a larger tire diameter travels farther per rotation, making your speedometer read slower than your true speed.
Does a tire size change affect fuel economy, not just my speedometer?+
It can. Larger, heavier tires increase rotating mass and rolling resistance, which can modestly reduce real-world fuel economy. They also change your effective gearing, since the engine now turns fewer revolutions for the same distance traveled — which can affect both MPG and how the vehicle feels off the line.
What does 'plus sizing' mean?+
Plus sizing means fitting a larger-diameter wheel with a lower-profile (lower aspect-ratio) tire so the tire's outer diameter stays close to the original — 'plus one' is one inch larger wheels, 'plus two' is two inches larger, and so on. Use this calculator to confirm the resulting diameter stays within a few percent of stock before buying.
Found this useful? Share it
Help someone else out — or link to it from your own site.
Link to this tool
Writing about tire size calculator? Paste this on your site to link here.
<a href="https://everyfix.net/tools/tire-size-calculator/">Tire Size Calculator</a> by <a href="https://everyfix.net">EveryFix</a>Related Tools
MPG Calculator
Calculate your vehicle's real-world fuel economy in MPG or L/100km.
Use tool →
Fuel Cost Calculator
Calculate the fuel cost for any trip based on distance and fuel economy.
Use tool →
Gas vs EV Cost Calculator
Compare annual fuel costs between a gas vehicle and an electric vehicle.
Use tool →
EV Charging Cost Calculator
Estimate how much it costs to charge your electric vehicle.
Use tool →