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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 without recalibrating, so you know your actual speed.
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
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. Conversely, slightly taller tires lower engine RPM at speed, which can offset some of the penalty.
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.