| Metric | Tire 1 | Tire 2 | Difference |
|---|---|---|---|
| Diameter | '+d1.toFixed(2)+' in | '+d2.toFixed(2)+' in | '+(d2-d1).toFixed(2)+' in ('+diff.toFixed(1)+'%) |
| Width | '+w1+' mm | '+w2+' mm | '+(w2-w1)+' mm |
| Sidewall | '+s1.toFixed(2)+' in | '+s2.toFixed(2)+' in | '+(s2-s1).toFixed(2)+' in |
| Circumference | '+c1.toFixed(2)+' in | '+c2.toFixed(2)+' in | '+(c2-c1).toFixed(2)+' in |
| Revs/Mile | '+rev1.toFixed(2)+' | '+rev2.toFixed(2)+' | '+(rev2-rev1).toFixed(2)+' |
1. Sidewall = (Width * Aspect Ratio / 100) / 25.4
2. Diameter = (Sidewall * 2) + Wheel Diameter
3. Circumference = Diameter * π
4. Revs/Mile = 63,360 / Circumference
Using the Tire Calculator
Choosing the right tires for your vehicle involves more than just picking a brand. Our tire calculator is designed to help you compare two different tire sizes to understand how a change in width, aspect ratio, or wheel diameter affects your vehicle's performance and speedometer accuracy.
Whether you are "plus-sizing" your rims for better aesthetics or looking for a narrower tire for winter driving, this tool provides the exact dimensions and differences you need to make an informed decision.
- Tire Width
- The width of the tire from sidewall to sidewall in millimeters. A higher number means a wider tire footprint.
- Aspect Ratio
- The height of the sidewall expressed as a percentage of the width. A "45" means the sidewall height is 45% of the tire's width.
- Wheel Diameter
- The diameter of the wheel (rim) in inches. This is the inner diameter of the tire.
How It Works
The tire calculator uses standard P-Metric tire geometry formulas to convert the three standard tire size numbers into physical dimensions. The most critical calculation is the Total Diameter, as this dictates how many times the tire rotates over a mile.
Total Diameter = ((Width × Aspect Ratio / 100) × 2 / 25.4) + Wheel Diameter
- Sidewall Height: Calculated by multiplying width by aspect ratio. We divide by 25.4 to convert millimeters to inches.
- Circumference: Found by multiplying the total diameter by Pi (π). This is the distance the tire travels in one full revolution.
- Revolutions per Mile: There are 63,360 inches in a mile. By dividing this by the circumference, we find how many times the tire spins per mile.
- Speedometer Error: Vehicles calculate speed based on tire revolutions. If you install a larger tire, it covers more ground per spin, making your speedometer read lower than your actual speed.
Tire Comparison Example
Example: A driver wants to move from a factory 215/55R17 tire to a sporty 235/45R18 setup.
Step-by-step solution for Tire 1:
- Width = 215mm; Ratio = 55%; Rim = 17in
- Sidewall (mm) = 215 * 0.55 = 118.25mm
- Sidewall (in) = 118.25 / 25.4 = 4.66in
- Total Diameter = (4.66 * 2) + 17 = 26.31in
- Circumference = 26.31 * 3.14159 = 82.66in
- Resulting Revs/Mile = 63,360 / 82.66 = 766.5
If Tire 2 results in a diameter of 26.33in, the difference is only 0.1%, meaning the speedometer remains accurate. Generally, a difference of less than 3% is considered safe for most vehicles.
Common Tire Questions
Will a wider tire fit my car?
While the tire calculator shows the mathematical dimensions, it does not account for physical clearances. You must check that the wider tire does not rub against the suspension struts, fenders, or wheel wells when the steering wheel is turned to full lock.
How does tire size affect fuel economy?
Generally, wider tires have more rolling resistance and weight, which can slightly decrease fuel economy. However, a taller tire effectively changes your gear ratio, which might improve highway fuel economy by lowering engine RPMs at cruising speeds.
Does tire diameter affect my ABS or Traction Control?
Modern vehicles rely on wheel speed sensors for ABS and Traction Control. If you change the tire size significantly (especially if you only change the rear tires and not the fronts), the computer may detect a discrepancy in wheel speeds and trigger a warning light. It is best to keep the overall diameter within 3% of the original factory specification.