Tire Size Calculator
Calculate tire dimensions, compare two tire sizes, and see how changing your tire size affects your speedometer. Enter tire sizes in the standard metric format (e.g., 225/45R17) and get instant results. Perfect for tire upgrades, replacements, and fitment checks.
How Tire Sizes Work
Tire sizes are standardized using the format: Width / Aspect Ratio R Rim Diameter. For example, in a 225/45R17 tire:
- 225 — Section width in millimeters
- 45 — Aspect ratio (sidewall height as a percentage of width)
- R — Radial construction
- 17 — Rim diameter in inches
Our calculator computes all the key tire dimensions:
Sidewall Height = (Width × Aspect Ratio) ÷ 100
Overall Diameter = (Sidewall Height × 2) + Rim Diameter
Circumference = Overall Diameter × π
Revolutions per Mile = 63,360 ÷ Circumference (in inches)
Why Tire Size Matters
- Speedometer Accuracy: Larger diameter tires cause the speedometer to read slower than actual speed. Smaller tires cause it to read faster.
- Vehicle Clearance: Larger tires may rub against wheel wells, suspension components, or fenders.
- Fuel Efficiency: Larger, heavier tires can reduce fuel economy.
- Ride Quality: Lower profile tires (smaller aspect ratio) provide better handling but a rougher ride.
- Gearing: Larger tires effectively change your final drive ratio, affecting acceleration and RPMs.
Speedometer Error Explained
When you change tire sizes, your speedometer reading will change. The speedometer is calibrated based on the original tire's circumference. If you install a tire with a different diameter:
- Larger Diameter: Speedometer reads slower than actual speed (you're going faster than indicated).
- Smaller Diameter: Speedometer reads faster than actual speed (you're going slower than indicated).
Example: If your speedometer reads 60 mph with larger tires, your actual speed might be 63 mph — a 5% difference that could lead to speeding tickets.
❓ Tire Size Calculator FAQ
What do the numbers on a tire mean?
A tire size like 225/45R17 means: 225 = width in mm, 45 = aspect ratio (% of width), R = radial, 17 = rim diameter in inches.
How do I calculate tire diameter?
Overall diameter = (Width × Aspect Ratio ÷ 100 × 2 ÷ 25.4) + Rim Diameter. Or simply: sidewall × 2 + rim.
How do I calculate tire circumference?
Circumference = Diameter × π (3.14159). For a 225/45R17 tire with a 24.97" diameter, the circumference is about 78.45".
How do I calculate revolutions per mile?
Revolutions per mile = 63,360 ÷ Circumference (in inches). For example, 63,360 ÷ 78.45 = 807.6 revolutions per mile.
How does changing tire size affect my speedometer?
Larger diameter tires make your speedometer read slower than your actual speed. Smaller diameter tires make it read faster. A 5% change in diameter causes a 5% change in speedometer reading.
Can I put larger tires on my car?
You can, but you need to check for clearance issues (fender, suspension, brake components) and consider speedometer error, gearing changes, and potential ABS/traction control issues. Always stay within 3% of the original diameter for safety.
What is the aspect ratio of a tire?
The aspect ratio is the sidewall height expressed as a percentage of the tire's width. For example, a 225/45 tire has a sidewall height that is 45% of 225 mm.
What is the difference between metric and standard tire sizes?
Metric tire sizes use millimeters for width and aspect ratio (e.g., 225/45R17). Standard or "flotation" sizes use inches for overall diameter and width (e.g., 31×10.50R15). This calculator focuses on metric sizes.
How do I convert tire width from mm to inches?
Divide the width in mm by 25.4. For example, 225 mm ÷ 25.4 = 8.86 inches.
What is the rolling circumference factor?
Under load, a tire's circumference is slightly reduced due to deflection. The rolling factor (typically 96%) adjusts the theoretical circumference to a more realistic rolling circumference.
Why is it important to stay within 3% diameter difference?
A diameter difference of more than 3% can cause significant speedometer error, affect ABS and traction control systems, and lead to clearance issues. Always consult with a tire professional for major changes.
How does tire size affect gas mileage?
Larger, heavier tires increase rolling resistance and unsprung weight, which can reduce fuel economy by 1-3 MPG. The reduced effective gearing also causes the engine to work harder at highway speeds.
What is the difference between plus sizing and minus sizing?
Plus sizing means increasing rim diameter while decreasing sidewall height to maintain overall diameter. Minus sizing means decreasing rim diameter while increasing sidewall height for better ride quality.
How do I choose the right tire size for my vehicle?
Always start with your vehicle's recommended tire size (found on the door jamb sticker or in the owner's manual). If you want to change sizes, stay within 3% of the original diameter and consult a tire professional.
What is the tire load index and speed rating?
The load index indicates how much weight a tire can carry. The speed rating indicates the maximum safe speed for the tire. These are separate from the tire size and are also important for safety.
How do I find my tire size?
Your tire size is printed on the sidewall of your tire in the format 225/45R17. You can also find it on the driver's door jamb sticker or in your owner's manual.
Can I mix tire sizes on my vehicle?
It's generally not recommended to mix tire sizes. For AWD and some performance vehicles, using different sizes can damage the drivetrain. Always use the same size on all four wheels unless your vehicle specifically requires staggered sizes.
What is the difference between a 225/45R17 and a 225/50R17?
The 225/50R17 has a taller sidewall (50% vs 45% of 225mm), making the overall diameter larger. This will raise the vehicle slightly and cause the speedometer to read slower than actual speed.
How accurate is this tire size calculator?
This calculator provides accurate results based on standard tire size formulas. However, actual tire dimensions can vary slightly between manufacturers. Always verify fitment with your tire provider.