Tire Size Comparison Calculator

The Formula

Tire overall diameter is calculated by adding twice the sidewall height to the rim diameter. The speedometer reads based on the original tire circumference, so any change in diameter introduces a speedometer error.

Variable Definitions

How to Use This Calculator

1. 1

   Enter your original tire size (the three numbers from your tire's sidewall, e.g., 225/50R17).

2. 2

   Enter the new tire size you are considering.

3. 3

   The calculator shows diameter difference and speedometer error.

4. 4

   A diameter difference within ±3% is generally acceptable for street use.

5. 5

   If the difference exceeds 3%, consider a different tire size to maintain safety and accuracy.

Common Applications

• Checking if a new tire size will fit your vehicle without rubbing fenders or suspension components
• Determining speedometer error when upgrading to larger or smaller tires for off-road or aesthetic purposes
• Comparing plus-size tire and wheel combinations to maintain safe handling, ABS function, and traction control accuracy

Tire dimensions — overall diameter equals rim diameter plus twice the sidewall height

Understanding the Concept

When you change tire sizes, the overall diameter changes — which means the number of revolutions per mile changes. Your speedometer is calibrated to your original tire diameter, so a larger tire makes the speedometer read lower than your actual speed. A difference of ±3% is generally within acceptable limits and may not require speedometer recalibration. Larger differences can also affect ABS, traction control, and odometer accuracy. Real-world example: upgrading from 225/50R17 to 235/45R18 (a common plus-one size). Original diameter = 17 × 25.4 + 2 × (225 × 0.50) = 656.8 mm. New diameter = 18 × 25.4 + 2 × (235 × 0.45) = 668.7 mm. Difference = 1.8% — within the safe 3% range. However, changing to 245/40R19 (plus-two) gives a diameter of 678.6 mm, a 3.3% difference — potentially large enough to trigger ABS and traction control issues on some vehicles. For off-road vehicles, increasing tire diameter is common for ground clearance, but the speedometer error can be significant: going from 31-inch to 33-inch tires (6.5% increase) means at an indicated 60 mph you are actually traveling at 64 mph, which could attract speeding tickets on highways.

Frequently Asked Questions

Sources & References