Last updated: March 2026
What Is the Tire Size Calculator?
The Tire Size Calculator is a free tool that decodes tire size numbers (like 225/65R17) into actual measurements — overall diameter, sidewall height, section width, circumference, and revolutions per mile. Over 300,000 people search for tire size calculators monthly when shopping for replacement tires, upsizing for off-road use, or verifying fitment. The comparison mode shows exactly how two tire sizes differ in diameter, width, and speedometer accuracy — essential information before buying new tires.
Every number on the side of your tire tells you something specific about its dimensions. The first number is the section width in millimeters — the distance from one sidewall to the other at the widest point. The second number is the aspect ratio, which tells you the sidewall height as a percentage of the width. "R" stands for radial construction (nearly all modern tires), and the last number is the wheel diameter in inches.
How to Read Tire Size Numbers
Enter a tire size like 225/65R17. The first number (225) is the tread width in millimeters. The second number (65) is the aspect ratio — the sidewall height is 65% of the width. "R" means radial construction. The last number (17) is the wheel diameter in inches. The calculator converts everything to understandable measurements and shows a visual cross-section diagram.
For example, a 225/65R17 tire has a section width of 8.86 inches, a sidewall height of 5.76 inches (225mm x 0.65), and an overall diameter of 29.52 inches. The overall diameter is calculated by adding two sidewall heights to the wheel diameter: (5.76 x 2) + 17 = 29.52 inches. This is the number that matters most for fitment, speedometer accuracy, and clearance.
Comparing Tire Sizes: What Changes?
When you change tire sizes, three things are affected: speedometer accuracy, ground clearance, and ride quality. A larger-diameter tire covers more ground per revolution, which makes your speedometer read slower than your actual speed. A common rule of thumb is to stay within 3% of the original tire's overall diameter to avoid significant speedometer error.
Wider tires generally offer better grip and handling due to a larger contact patch, but they also increase rolling resistance and can reduce fuel economy. Taller sidewalls provide a smoother ride and better impact absorption for potholes, while shorter sidewalls (lower aspect ratio) improve cornering response and steering feel. The comparison tool shows you exactly how any two sizes differ across every measurement.
Speedometer Error and Tire Size
Your vehicle's speedometer measures wheel rotations and calculates speed based on the original tire's circumference. When you install larger tires, each revolution covers more distance, so your actual speed is higher than what the speedometer shows. Conversely, smaller tires make the speedometer read fast. A 3% change in diameter produces a 3% speedometer error — at 60 mph indicated, you'd actually be traveling at 61.8 mph or 58.2 mph.
Most states allow up to a 3-5% speedometer error before it becomes a legal concern. Beyond that, your odometer and trip computer will also be inaccurate, affecting reported fuel economy and total mileage. Some modern vehicles allow speedometer recalibration through the dealer, while aftermarket tuners can also adjust the calibration for vehicles with modified tire sizes.
Frequently Asked Questions
What do tire size numbers mean?
In 225/65R17: 225 is the tread width in millimeters, 65 means the sidewall height is 65% of the width, R indicates radial construction, and 17 is the wheel diameter in inches. The calculator converts this into overall diameter, height, circumference, and revolutions per mile.
How does tire size affect my speedometer?
Larger tires cover more ground per revolution, making your speedometer read slower than your actual speed. A 3% increase in tire diameter means at an indicated 60 mph, you're actually going about 61.8 mph. The comparison tool calculates this exactly for any two tire sizes.
Can I put a different size tire on my car?
Generally you can go 3% larger or smaller in overall diameter without significant issues. Larger changes may affect speedometer accuracy, clearance, and handling. Use the comparison mode to check the exact difference in diameter and speedometer error before buying.
What's the difference between 225/65R17 and 235/65R17?
The first is 225mm wide, the second is 235mm wide (10mm wider tread). Because the aspect ratio is the same (65%), the wider tire also has a slightly taller sidewall, resulting in a slightly larger overall diameter. Enter both sizes in the calculator to see the exact measurements.
What size tire is equivalent to 33 inches?
Common '33-inch' tire sizes include 285/75R16 (32.8"), 275/70R17 (32.2"), and 285/70R17 (32.7"). Enter any size in the calculator to see the exact overall diameter and whether it matches your target height.