Tire Size Comparison Calculator
Compare two tire sizes side by side — diameter, circumference, sidewall height, revolutions per mile, and speedometer error.
Tire 1 / Original
Tire 2 / New
How Tire Sizes Work
A tire size like 265/70R17 encodes three measurements. The first number (265) is the section width in millimeters. The second number (70) is the aspect ratio — the sidewall height as a percentage of the width. The letter R means radial construction, and the last number (17) is the rim diameter in inches.
Calculating Overall Diameter
The sidewall height in inches is:
Sidewall = Width (mm) × (Aspect Ratio / 100) / 25.4
Since the tire has a sidewall on both the top and bottom of the wheel, the overall diameter is:
Diameter = (Sidewall × 2) + Rim Diameter
For a 265/70R17: sidewall = 265 × 0.70 / 25.4 = 7.30", so diameter = 7.30 × 2 + 17 = 31.61".
Circumference & Revolutions per Mile
Once you know the diameter, circumference is simply π × diameter. Revolutions per mile divides the number of inches in a mile (63,360) by the circumference:
Revs/mile = 63,360 / (π × Diameter)
Larger tires have fewer revolutions per mile, which affects gearing, fuel economy, and speedometer accuracy.
Speedometer Impact
Your speedometer is calibrated for the original tire size. When you install larger tires, each wheel revolution covers more ground, so your actual speed is higher than what the speedometer reads. The error percentage is:
Speedo Error (%) = ((New Diameter − Original Diameter) / Original Diameter) × 100
A positive value means the speedometer reads low (you are going faster than displayed). A negative value means it reads high. Most vehicles tolerate up to about 3% error before recalibration is recommended.
Common Uses
- Upsizing tires: Will larger tires fit without rubbing? How much taller will the vehicle sit?
- Speedometer correction: How far off will my speedo be with the new size?
- Gear ratio planning: Bigger tires lower your effective gear ratio — pair this with the gear ratio calculator to find the right axle ratio.
- Plus sizing: Going to a larger rim with a lower profile tire while keeping the same overall diameter.
Frequently Asked Questions
How do I read a tire size like 265/70R17?
The first number (265) is the tread width in millimeters. The second number (70) is the aspect ratio — the sidewall height as a percentage of the width. R means radial construction, and 17 is the wheel diameter in inches. So a 265/70R17 has a 265 mm wide tread with a sidewall that is 70% of 265 mm (185.5 mm) on a 17-inch wheel.
Will larger tires affect my speedometer?
Yes. A taller tire covers more ground per revolution, so your speedometer will read lower than your actual speed. For example, going from a 28-inch to a 30-inch tire means your actual speed is about 7% higher than displayed. Use our Speedometer Correction calculator to find the exact error.
How much bigger can I go without rubbing?
As a general rule, you can increase overall tire diameter by about 3% (roughly 1 inch) without rubbing on most vehicles. Going beyond that usually requires fender trimming, wheel spacers, or a lift. Always check clearance at full steering lock and full suspension compression before driving.
What is the difference between plus sizing and upsizing tires?
Plus sizing increases the wheel diameter while decreasing the sidewall height to keep the overall diameter roughly the same. Upsizing increases the overall tire diameter. Plus sizing maintains speedometer accuracy and improves handling, while upsizing raises ground clearance but affects your speedometer reading.
Related Articles
How to decode tire size numbers, what happens when you change tire sizes, and how it affects speedometer accuracy, gearing, clearance, and ride quality.
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Related Calculators
Calculate speedometer error after changing tire sizes. Find your actual speed vs. displayed speed and the percentage difference.
Wheel OffsetConvert between wheel offset (mm) and backspace (inches). Calculate poke or tuck when changing wheel width or offset.
Bolt PatternCalculate bolt pattern (PCD) from lug count and bolt-to-bolt measurement. Identify your wheel bolt pattern for 4, 5, 6, and 8 lug wheels.
Gear RatioCalculate vehicle speed from RPM, gear ratio, final drive, and tire size — or find RPM at a given speed. Essential for gear selection and tire sizing.