Tire Size Comparison Calculator

Enter Tire Sizes

How Tire Size Comparison Works

Every metric tire size follows the format Width/AspectRatio R RimDiameter - a notation standardized by the Tire and Rim Association (TRA) in its annual yearbook and codified globally under the European Tyre and Rim Technical Organisation (ETRTO) standards. The three numbers encode everything you need to calculate the tire's physical dimensions.

The section width (first number) is the distance in millimeters from the widest point of one sidewall to the other when the tire is mounted on a specified-width rim and inflated to rated pressure - but carrying no load. The aspect ratio (second number) is the sidewall height expressed as a percentage of the section width. The rim diameter (last number) is the wheel's bead seat diameter in inches. SAE J1100 - the Society of Automotive Engineers' standard for motor vehicle dimensions - defines how these measurements are taken under controlled conditions: the tire must be mounted on its measuring rim, inflated to the specified pressure, and allowed to stabilize for at least 24 hours before measurement.

Flotation sizes like 33x12.50R15 use a different notation common on light truck and off-road tires. The first number is overall diameter in inches, the second is section width in inches, and the third is rim diameter in inches. Both systems describe the same physical tire - just with different notation. This calculator handles both and converts between them so you can compare a metric tire against a flotation-sized tire directly.

The Math Behind It

For a metric tire size W/A R D, the core formulas are:

• Sidewall Height = Width × (Aspect Ratio ÷ 100). For a 265/70R17: 265 × 0.70 = 185.5 mm (7.30″).
• Overall Diameter = (2 × Sidewall Height) + (Rim Diameter × 25.4). Both sidewalls plus the rim: (2 × 185.5) + (17 × 25.4) = 371.0 + 431.8 = 802.8 mm (31.61″).
• Circumference = π × Overall Diameter. The rolling distance per revolution: π × 802.8 = 2,522.1 mm (99.29″).
• Revolutions per Mile = 1,609,344 ÷ Circumference (in mm). Since one mile equals 1,609,344 mm: 1,609,344 ÷ 2,522.1 = 638 revs/mile.
• Speedometer Correction = (New Circumference ÷ Original Circumference). Actual speed equals indicated speed multiplied by this ratio.

These formulas calculate the theoretical dimensions based on the tire designation. Actual measured dimensions can vary by 2–4% depending on the manufacturer, tread design, and inflation pressure. The TRA yearbook publishes tolerances for each size - for example, a 265/70R17 has a specified overall diameter range rather than a single exact figure.

Industry Standards & References

Several standards govern how tire dimensions are specified, measured, and compared:

• SAE J1100 - Motor Vehicle Dimensions. Defines measurement procedures for vehicle and tire dimensions, including loaded radius, overall diameter, and section width under specified conditions.
• SAE J2047 - Tire Performance Technology. Covers tire labeling, sizing, and performance metrics including rolling circumference data.
• TRA Yearbook - Published annually by the Tire and Rim Association. Lists every approved tire size with its design rim width, overall diameter range, section width range, and load/inflation tables. This is the authoritative reference for tire fitting in North America.
• ETRTO Standards Manual - The European equivalent of the TRA yearbook. Provides metric tire dimension standards used internationally.
• FMVSS 109 & 139 - Federal Motor Vehicle Safety Standards for passenger and light truck tires. These regulate minimum performance requirements including dimensional compliance, endurance, and high-speed performance.

When automakers specify a tire size in the owner's manual, they reference the TRA/ETRTO standardized dimensions. Your vehicle's fender clearance, suspension geometry, speedometer calibration, and gear ratios are all engineered around those specified dimensions.

Step-by-Step Example

Scenario: You own a 2020 Toyota Tacoma TRD Off-Road with stock 265/70R17 tires. You want to upgrade to 285/70R17 for wider tread and more ground clearance. Here's exactly how the numbers compare:

Step 1 - Calculate Tire 1 (265/70R17) dimensions:

• Sidewall: 265 × 0.70 = 185.5 mm (7.30″)
• Overall diameter: (2 × 185.5) + (17 × 25.4) = 802.8 mm (31.61″)
• Circumference: π × 802.8 = 2,522.1 mm (99.29″)
• Revolutions per mile: 1,609,344 ÷ 2,522.1 = 638

Step 2 - Calculate Tire 2 (285/70R17) dimensions:

• Sidewall: 285 × 0.70 = 199.5 mm (7.85″)
• Overall diameter: (2 × 199.5) + (17 × 25.4) = 830.8 mm (32.71″)
• Circumference: π × 830.8 = 2,610.0 mm (102.76″)
• Revolutions per mile: 1,609,344 ÷ 2,610.0 = 617

Step 3 - Compare the differences:

• Diameter increase: +28.0 mm (+1.10″) - that's a 3.5% increase
• Width increase: +20 mm (+0.79″) - 7.5% wider footprint
• Sidewall increase: +14.0 mm (+0.55″) - taller sidewall absorbs more impact
• Ground clearance gain: +14.0 mm (+0.55″) - half the diameter increase lifts the axle

Result - Speedometer impact: The speedometer correction ratio is 2,610.0 ÷ 2,522.1 = 1.0349. When your speedometer reads 65 mph, your actual speed is 67.3 mph - you're going 2.3 mph faster than indicated. At 75 mph indicated, you're actually traveling 77.6 mph. Your odometer will also under-report mileage by 3.5%, which affects service interval tracking. The 3.5% diameter increase exceeds the common 3% fitment guideline, meaning you may experience rubbing at full lock on a stock Tacoma without a leveling kit or fender trimming.

Common Mistakes to Avoid

• Ignoring the width change when only wanting more height: Going from 265/70R17 to 285/70R17 adds 20 mm (0.79″) of width per side. That extra width can cause rubbing on the upper control arm or inner fender liner during full suspension compression - even if the diameter increase seems modest. Why it matters: If you only need more height, a 265/75R17 adds diameter without increasing width.
• Assuming nominal size equals actual size: A "285" mm wide tire may actually measure 280–295 mm depending on manufacturer. BFGoodrich KO2 in 285/70R17 measures approximately 32.8″ overall, while a Falken Wildpeak A/T3W in the same size measures closer to 32.6″. Always check the manufacturer's published specifications for the specific tire model. Why it matters: Two tires with the same size designation can have meaningfully different actual dimensions.
• Forgetting to recalibrate the speedometer: A 3.5% error at highway speed means you're traveling 2.3 mph faster than shown at 65 mph. Over time this adds up to speeding tickets, incorrect odometer readings, and misaligned service intervals. Why it matters: Most modern trucks can be recalibrated via a dealer reflash or an aftermarket programmer. Some states require accurate speedometer readings for safety inspection.
• Not checking backspacing and offset: A wider tire on the same wheel may push the tire outward past the fender, or inward toward suspension components. The tire's section width is only part of the equation - wheel offset (ET) determines where the tire sits relative to the hub face. Why it matters: Running a wider tire on a stock wheel with stock offset can cause clearance problems that a simple size comparison won't reveal.
• Overlooking the effect on gear ratio: A taller tire effectively raises your final drive ratio. Going from 31.61″ to 32.71″ tires on a truck with 3.73:1 gears changes the effective ratio to approximately 3.73 × (31.61 ÷ 32.71) = 3.60:1. Why it matters: The taller tire makes the engine turn fewer revolutions per mile, reducing low-end torque response and potentially causing the transmission to hunt between gears. Re-gearing to 4.10:1 is a common correction.

When to Use This Calculator

• Upgrading tire size on a truck or SUV: Before you buy, verify the diameter difference stays within your vehicle's clearance limits and understand the speedometer error you'll be living with.
• Cross-referencing metric and flotation sizes: When you're comparing a 285/75R16 against a 33x12.50R15, this calculator converts both to the same dimensions so you can make a direct side-by-side comparison.
• Planning a wheel-and-tire package: If you're changing rim size (e.g., going from 17″ to 18″ wheels), use this tool to find a tire size on the new rim that matches your original overall diameter - keeping your speedometer accurate and avoiding clearance issues.
• Verifying speedometer correction after a tire change: If you've already installed larger tires and suspect your speedometer is off, enter your old and new sizes to see the exact correction factor and decide whether a recalibration is worthwhile.