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Tire Diameter Equation
Enter two tire sizes to compare their diameter, circumference, sidewall height, and revolutions per mile.
D = 2 × (W × AR / 2540) + Wd
How It Works
Enter two tire sizes (e.g., 225/45R17 and 235/40R18) to compare their diameter, circumference, sidewall height, and revolutions per mile. The calculator also shows how your speedometer reading changes when you switch tire sizes.
Example Problem
You switch from 225/45R17 to 235/40R18. At 60 mph on the speedometer, what is your actual speed?
- Original diameter: (2 × 225 × 0.45 / 25.4) + 17 = 24.97 in
- Upgrade diameter: (2 × 235 × 0.40 / 25.4) + 18 = 25.41 in
- Actual speed: (25.41 / 24.97) × 60 ≈ 61.1 mph
Key Concepts
Tire size designations like 225/45R17 encode three measurements: section width (225 mm), aspect ratio (45% of width = sidewall height), and wheel diameter (17 inches). Overall tire diameter = 2 × sidewall height + wheel diameter. Changing tire size affects speedometer accuracy proportionally — a larger diameter means the speedometer reads lower than actual speed because each wheel revolution covers more ground.
Applications
- Wheel upgrades: comparing plus-one or plus-two wheel/tire packages to maintain similar overall diameter and speedometer accuracy
- Off-road vehicles: evaluating how larger tires affect ground clearance, gear ratios, and speedometer calibration
- Fleet management: ensuring replacement tires stay within manufacturer diameter tolerances for ABS and traction control systems
- Motorsport: selecting tire sizes to optimize grip, sidewall stiffness, and rolling circumference for specific track conditions
Common Mistakes
- Exceeding the 3% diameter rule — going beyond 3% difference from OEM tire size can cause speedometer errors, ABS malfunctions, and wheel-well rubbing
- Ignoring load rating when upsizing — a wider, lower-profile tire is not always rated for the same load as the original tire
- Forgetting that aspect ratio is a percentage of width, not a fixed height — a 225/45 and a 245/45 have different sidewall heights (101 mm vs. 110 mm)
- Comparing only diameter without checking width — a tire that fits the diameter target may be too wide for the wheel or fender clearance
Frequently Asked Questions
How much bigger can I go on tire size?
Most manufacturers recommend staying within 3% of the original tire diameter. Going larger can affect speedometer accuracy, ABS calibration, and may cause rubbing in the wheel well.
What does the aspect ratio mean on a tire?
The aspect ratio is the sidewall height expressed as a percentage of the tire width. A 225/45 tire has a sidewall that is 45% of 225 mm, or about 101 mm (4 inches).
Does changing tire size affect fuel economy?
Yes. Larger, heavier tires increase rolling resistance, which can reduce fuel economy by 1–3%. Wider tires also create more aerodynamic drag at highway speeds.
Related Calculators
- Percent Difference Calculator — compare two tire diameter measurements.
- Battery Life Calculator — estimate discharge time for automotive and deep-cycle batteries.
- Trailer Towing Calculator — check tow vehicle capacity with your tire setup.
- Gear Equations Calculator — see how tire size changes affect effective gear ratio and speed.
- Length Unit Converter — convert tire measurements between mm, inches, and cm.
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