Tire Ratio Calculator
Use this calculator to compare two different tire sizes and understand how changing your tire dimensions affects your vehicle's speedometer, effective gearing, and overall diameter. This is crucial for maintaining accuracy and ensuring proper fitment.
Old Tire Size (Current)
New Tire Size (Proposed)
Reference Speed
Understanding Tire Ratios and Their Impact
Changing your vehicle's tire size is a common modification, whether for aesthetic reasons, performance upgrades, or simply replacing worn-out tires with a different specification. However, altering tire dimensions can have significant effects on your vehicle's performance, safety, and even legal compliance.
How to Read Tire Sizes
Tire sizes are typically displayed in a format like "205/55R16":
- 205: This is the tire's width in millimeters (mm) from sidewall to sidewall.
- 55: This is the aspect ratio, representing the sidewall height as a percentage of the tire's width. In this case, the sidewall height is 55% of 205mm.
- R: Denotes radial construction, the most common type of tire.
- 16: This is the diameter of the wheel (rim) in inches that the tire is designed to fit.
Key Calculations Explained
Our Tire Ratio Calculator performs several important calculations:
- Overall Diameter: This is the total height of the tire from the ground to the top. A larger diameter tire will cover more ground with each revolution.
- Revolutions Per Mile (RPM): This indicates how many times the tire spins to travel one mile. A larger diameter tire will have fewer revolutions per mile.
- Speedometer Error: Your vehicle's speedometer is calibrated based on the original tire size. Changing the tire diameter will cause your speedometer to read inaccurately. If your new tires are larger, your speedometer will read lower than your actual speed. If they are smaller, it will read higher. This can lead to speeding tickets or misjudging your speed.
- Effective Gear Ratio Change: The tire's diameter directly impacts your vehicle's effective gear ratio. A larger tire effectively "raises" your gearing (like having a numerically lower gear ratio), which can reduce acceleration but potentially improve fuel economy at highway speeds. A smaller tire "lowers" your gearing (like having a numerically higher gear ratio), improving acceleration but potentially increasing RPMs at highway speeds.
Why is this important?
- Safety: An inaccurate speedometer can be dangerous, leading to unintended speeding or misjudging stopping distances.
- Performance: Changes in effective gear ratio can affect acceleration, towing capacity, and fuel efficiency.
- Fitment: A significantly larger tire might rub against wheel wells, suspension components, or fender liners, especially during turns or when the suspension compresses.
- ABS/Traction Control: Modern vehicles rely on precise wheel speed sensor readings. Significant changes in tire diameter can confuse these systems, potentially leading to malfunction or reduced effectiveness.
Always consult your vehicle's manufacturer recommendations and consider professional advice before making significant changes to your tire size.
Calculation Results:
"; resultsHtml += "Old Tire Overall Diameter: " + oldOverallDiameter.toFixed(2) + " inches"; resultsHtml += "New Tire Overall Diameter: " + newOverallDiameter.toFixed(2) + " inches"; resultsHtml += "Diameter Difference: " + diameterDifferencePercent.toFixed(2) + "%"; resultsHtml += ""; resultsHtml += "Old Tire Revolutions Per Mile: " + oldRevsPerMile.toFixed(2) + ""; resultsHtml += "New Tire Revolutions Per Mile: " + newRevsPerMile.toFixed(2) + ""; resultsHtml += "
"; resultsHtml += "Speedometer Reading: If your speedometer reads " + referenceSpeed.toFixed(0) + " MPH,"; resultsHtml += "your actual speed with the new tires will be " + actualSpeed.toFixed(2) + " MPH."; resultsHtml += "This is a " + speedometerErrorPercent.toFixed(2) + "% speedometer error."; resultsHtml += "
"; resultsHtml += "Effective Gear Ratio Change: " + effectiveGearRatioChangePercent.toFixed(2) + "%" + gearRatioDescription + ""; resultDiv.innerHTML = resultsHtml; }