What Is a Tire Size Calculator?
A tire size calculator decodes the numbers on your tire sidewall and computes key dimensions including overall diameter, circumference, section height, and revolutions per mile. When comparing two tire sizes, it also shows the diameter difference and the resulting speedometer error, helping you make informed decisions when changing tire sizes.
Choosing the wrong tire size can affect your speedometer accuracy, odometer readings, fuel efficiency, handling, and clearance inside the wheel well. This calculator helps you verify compatibility before purchasing.
What This Calculator Does
Enter the three numbers from your tire code (width, aspect ratio, rim diameter) for both your original and new tire sizes. The calculator displays all key measurements and compares the two sizes side by side.
- Inputs: Tire width (mm), aspect ratio (%), rim diameter (inches) for original and new tires
- Outputs: Overall diameter, circumference, revolutions per mile, section height, diameter difference, speedometer error percentage
How to Read a Tire Code
A standard tire code like 225/50R17 contains three key numbers:
- 225 = Section width in millimeters (the width of the tire from sidewall to sidewall)
- 50 = Aspect ratio (the sidewall height as a percentage of the section width)
- 17 = Rim diameter in inches (the diameter of the wheel the tire fits)
How the Calculation Works
Section Height (mm) = Width x (Aspect Ratio / 100)
Overall Diameter (in) = Rim Diameter + (2 x Section Height / 25.4)
Circumference (in) = pi x Overall Diameter
Revs per Mile = 63,360 / Circumference
The section height is calculated from the width and aspect ratio, then converted to inches and added to the rim diameter (multiplied by 2 since there are two sidewalls) to get the overall tire diameter. Circumference follows from the diameter, and revolutions per mile is derived by dividing the number of inches in a mile (63,360) by the tire circumference.
How to Use the Calculator
- Find your current tire size on the sidewall of your tire (e.g. 225/50R17)
- Enter the three values into the Original Tire fields
- Enter the new tire size you are considering into the New Tire fields
- Review the dimension breakdown for each tire
- Check the comparison panel for diameter difference and speedometer error
Example Calculations
Example 1: Plus-Sizing Wheels
Original: 225/50R17. New: 235/45R18. The overall diameter of 225/50R17 is approximately 26.9 inches. The 235/45R18 is approximately 26.3 inches. The diameter difference is 0.6 inches (2.2%), resulting in a speedometer reading about 2.2% higher than actual speed, which is within the acceptable range.
Example 2: Off-Road Upgrade
A truck owner swaps from 265/70R17 (31.6 inches) to 285/75R17 (33.8 inches). The 6.8% diameter increase means the speedometer reads about 6.8% lower than actual speed, requiring recalibration to maintain accurate readings.
Real-World Scenarios
Fitting Larger Wheels
Many car owners upgrade to larger diameter rims for appearance. Using a lower aspect ratio tire maintains the overall diameter and keeps speedometer accuracy. This is called "plus-sizing" and requires matching the total diameter as closely as possible.
Winter Tire Selection
When choosing winter tires, a narrower width improves snow traction by increasing contact pressure. A 205/55R17 winter tire can replace a 225/50R17 summer tire with only a minor diameter difference, keeping speedometer error within acceptable limits.
Checking Clearance
Larger tires need more clearance in the wheel well. Knowing the exact diameter and section width helps you verify that a new size will not rub against the fender, suspension components, or chassis during full lock or suspension travel.
Why This Calculation Matters
Installing the wrong tire size without checking dimensions can lead to speedometer errors, odometer inaccuracy affecting service intervals, rubbing in the wheel well, reduced clearance, changed handling characteristics, and even failure to meet legal road requirements in some jurisdictions.
Common Mistakes to Avoid
- Exceeding 3% diameter difference: Most manufacturers and tire experts recommend keeping the total diameter within 3% of the original to maintain acceptable speedometer accuracy
- Ignoring load rating: A tire's dimensions alone do not determine if it is safe. The load index must meet or exceed the vehicle manufacturer's requirement
- Not checking speed rating: The speed rating must be at least as high as the original tire's rating for safe highway use
- Mixing tire sizes: Never mix tire sizes on the same axle. If changing size, replace all four tires simultaneously or at minimum both on the same axle