31×10.5R15 to Metric: Your Complete Tire Size Conversion Guide

ByThe Answerlode Editorial Team

The Metric Equivalent: 265/75R15

If you need the metric version of a 31×10.50R15 tire, the answer is 265/75R15. This is the closest match in metric sizing, with only a 1.1% difference in overall diameter. That works out to roughly one-third of an inch—small enough that most drivers won’t notice any performance or speedometer drift.

How the Conversion Actually Works

Standard tire sizes (also called inch sizes) use a different measurement system than metric sizes. Here’s the breakdown:

  • 31×10.5R15: 31 inches diameter, 10.5 inches width, R (radial), 15-inch wheel
  • 265/75R15: 265 mm width, 75% aspect ratio, R (radial), 15-inch wheel

To convert the width: 10.5 inches × 25.4 mm per inch = 266.7 mm, which rounds to 265 mm in metric.

For the aspect ratio: Take the sidewall height (half the difference between tire diameter and wheel diameter), divide by width, and multiply by 100. For the 31×10.5R15, that’s (8 inches ÷ 10.5 inches) × 100 = 76.2%, which rounds down to 75%.

Detailed Measurement Comparison

Here’s how the two tires actually stack up against each other:

  • Overall diameter: 31×10.5R15 is 30.98 inches; 265/75R15 is 30.65 inches. Difference: 0.33 inches (8.4 mm)
  • Tread width: 31×10.5R15 is 10.51 inches; 265/75R15 is 10.43 inches. Difference: 0.08 inches (2 mm)
  • Sidewall height: 265/75R15 is 7.8 inches (199 mm)
  • Circumference: 265/75R15 is 96.4 inches, making 657 revolutions per mile

These tiny differences mean the 265/75R15 performs almost identically to the original.

What About Other Options Like 255/80R15?

You might find online suggestions for 255/80R15 as a conversion, but this is a less accurate match. The 255/80R15 produces a larger diameter and different aspect ratio, creating a noticeable change in speedometer accuracy and ride characteristics. If you’re forced to choose between what’s in stock, the 265/75R15 remains the superior option.

Availability and Fitment

The 265/75R15 is far more common in retail tire inventories than the 31×10.5R15 standard size, especially for vehicles like Jeeps and pickup trucks. It’s approved to fit wheels that are 7 to 9 inches wide. Before purchasing, verify that your wheel width falls within the manufacturer’s recommended range, and check your vehicle’s load rating requirements.

Why This Matters

Staying within the 1-2% diameter tolerance keeps your speedometer accurate and prevents transmission or drivetrain stress. A much larger tire diameter can trick your vehicle into thinking it’s going slower than it actually is.

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