🔵Tire Size Calculator
Calculate tire dimensions from a tire code (e.g. 265/70R17), compare two tire sizes side-by-side, or find the correct tire spec when changing to a new wheel (rim) size.
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Overall Diameter (mm)
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Tire Size Calculator
A tire size code like 265/70R17 encodes all the information needed to calculate a tire's physical dimensions. This calculator decodes those numbers, computes the full set of dimensions in both metric and imperial units, and compares two tire sizes side-by-side — including the speedometer error introduced by a diameter change.
How to read 265/70R17: section width 265 mm | aspect ratio 70% | radial construction | 17-inch rim
Understanding your tire's measurements is essential whenever you are replacing tires, switching to a different rim size, fitting aftermarket wheels, or checking clearance for a lift kit. The numbers molded into the sidewall follow a standardized format, and decoding them correctly prevents costly mistakes like ordering the wrong size or unknowingly affecting your vehicle's safety systems.
Decoding the Tire Size Code
| Part of Code | Example | What It Means |
|---|---|---|
| Section width | 265 | Width of the inflated tire sidewall-to-sidewall in mm |
| Aspect ratio | 70 | Sidewall height as % of section width (70% × 265 = 185.5 mm) |
| Construction | R | R = Radial (virtually all modern passenger tires) |
| Rim diameter | 17 | Wheel diameter in inches that the tire fits |
Why Overall Diameter Is the Critical Measurement
The overall diameter of a tire determines how far the vehicle travels per wheel revolution. This affects every calculation the vehicle's computer performs: speedometer reading, odometer accuracy, ABS wheel speed sensors, traction control thresholds, and transmission shift points. When you fit tires with a different overall diameter than the stock specification, all of these systems become inaccurate by the same percentage as the diameter change.
If the new tire is 3% larger in diameter, your speedometer will read 3% low — you will be travelling at 103 km/h when the gauge shows 100. While small variations are generally tolerable, changes above ±3% begin to noticeably affect real-world driving and may trigger warning lights on modern vehicles.
Plus-Sizing: Bigger Rims, Lower Profile
Plus-sizing — fitting a larger-diameter rim with a correspondingly lower-profile tire — is one of the most common aftermarket modifications. A larger rim with less sidewall improves steering response and cornering precision by reducing sidewall flex. The trade-off is a stiffer ride, less impact absorption, and greater susceptibility to pothole damage. The rule is to match the overall diameter of the original tire as closely as possible while changing the rim size, keeping the aspect ratio change to the nearest standard value (30, 35, 40, 45, 50, 55, 60, 65, 70, 75).
Speedometer Error and Correction
The speedometer error introduced by a tire size change is calculated as: Error % = (New Diameter − Old Diameter) ÷ Old Diameter × 100. At 100 km/h indicated, a +3% diameter increase means your actual speed is 103 km/h. Many aftermarket ECU tuning tools allow speedometer correction to compensate for tire size changes. Some GPS-based speedometer apps also provide a correction factor.
Comparing Two Tire Sizes: What to Check
- Overall diameter difference: Keep within ±3% for safety system accuracy; ±1% for odometer accuracy.
- Width difference: A wider tire may rub on fenders or suspension components at full lock — always check physical clearance.
- Load index and speed rating: Never fit tires with a lower load index or speed rating than the original specification.
- Revolutions per km/mile: Used to verify GPS and telematics system calibration after a tire change.
Frequently Asked Questions
What does the tire code 265/70R17 mean?
The code 265/70R17 means: section width of 265 mm (the tire's widest point), aspect ratio of 70% (the sidewall height is 70% of 265 mm = 185.5 mm), R for radial construction, and a 17-inch rim diameter. The overall diameter is the rim diameter (17 × 25.4 = 431.8 mm) plus twice the sidewall height (2 × 185.5 mm), totaling approximately 803 mm or 31.6 inches.
How much does a tire size change affect the speedometer?
Speedometer error equals the percentage change in overall tire diameter. If the new tire is 3% larger in diameter, your speedometer reads 3% low — you're going 103 km/h when it shows 100. Modern vehicles with ABS, traction control, and adaptive transmissions rely on accurate wheel speed data, so a change over ±3% may affect these systems. Some vehicles can be recalibrated by a dealer or with aftermarket ECU tools.
Can I fit wider tires without changing the rim size?
Yes, within limits. A wider tire on the same rim increases the section width and slightly changes the overall diameter. However, a tire that is too wide for its rim will have a bulging sidewall, compromising handling and safety. Tire manufacturers publish approved rim width ranges for each tire size — typically the rim should be 70–90% of the tire's section width. Always check clearance with the fender, suspension, and brake components, especially at full steering lock.
What is the maximum allowable tire size change?
Most vehicle manufacturers allow an overall diameter variation of ±3% from the OEM specification. Changes beyond ±5% can cause ABS and traction control malfunctions, affect speedometer accuracy significantly, and in some cases void vehicle warranties. For lifted trucks or SUVs running significantly larger tires, a speedometer correction recalibration and sometimes a differential regear are required to restore correct operation.
What is plus-sizing and why do people do it?
Plus-sizing means increasing the rim diameter by one or more inches while fitting a lower-profile tire to maintain the same overall diameter. For example, upgrading from 205/65R15 to 215/50R17 keeps the overall diameter nearly identical while fitting a 17-inch wheel. The benefit is improved handling and cornering precision due to less sidewall flex. The drawback is a firmer ride, increased vulnerability to pothole damage, and typically higher tire cost.
How do I find a tire size equivalent when changing rim size?
To find an equivalent tire size for a new rim diameter: calculate the target sidewall height (original sidewall height minus any change from the rim size increase), then divide by the desired section width and multiply by 100 to get the aspect ratio. Round to the nearest standard aspect ratio (30, 35, 40, 45, 50, 55, 60, 65, 70). Use this calculator's comparison mode — enter your original tire and the new rim size, and it will find the closest standard aspect ratio automatically.
Why are tire sizes measured in a mix of metric and imperial units?
Tire sizing is an artifact of the industry's history. Section width and sidewall height (derived from the aspect ratio) are measured in millimetres because the metric system was adopted for tire manufacturing in the mid-20th century. Rim diameter remained in inches because wheel manufacturing was already standardized in the US and UK with inch measurements. This mixed-unit convention is universal across all modern passenger, light truck, and SUV tires worldwide.