Ultimate Tire Size Calculator & Metric Conversion
Enter your tire size below to instantly calculate its overall diameter, section width, sidewall height, and circumference in both inches and millimeters. Find your exact metric-to-inch tire equivalent and shop live deals from top brands!
Enter Metric Tire Size
| Specification | Inches | Millimeters |
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How to Use the Metric to Inch Tire Size Calculator
Understanding your vehicle’s tire dimensions can be confusing, especially when switching between standard metric sizes and flotation (inch) sizes. Our advanced tire size calculator instantly converts any metric tire spec (like 275/65R18) into its exact inch equivalent.
Why Metric to Inch Conversion Matters?
If you drive a truck, Jeep, or SUV, you might be looking to upgrade to larger off-road or all-terrain tires. Most aftermarket suspension lifts and leveling kits state their clearances in inches (e.g., “Fits up to 33-inch tires”). However, factory tires are almost always written in metric format.
By using our tool, you can precisely calculate the:
Overall Diameter: Ensure the new tire fits your wheel well without rubbing.
Section Width: Measure the true footprint of your tread.
Revolutions Per Mile: Crucial data for calibrating your vehicle’s computer and speedometer.
Always remember that changing your tire’s overall diameter alters your gearing and [speedometer reading]. Once you find your perfect fitment, you can shop our massive inventory to find the best deals online.
Tire Speedometer Error Calculator
Changing your tire size affects how accurately your speedometer and odometer read. Enter your original (OEM) tire size and your new tire size below to see your actual speed at common speedometer readings.
Compare Two Tire Sizes Side-by-Side
See exactly how two tire sizes differ in overall diameter, width, sidewall height, circumference, and revolutions per mile.
Plus Sizing: How Rim Size Affects Overall Diameter
"Plus sizing" means fitting a larger-diameter wheel with a lower-profile tire while keeping the overall tire diameter close to stock. This keeps your speedometer accurate and your suspension geometry unchanged.
Use the comparison tool above to check that your plus-sized combination keeps overall diameter within about 3% of your original size — this protects your ABS and speedometer accuracy.
Tire Size Calculator: Frequently Asked Questions
Why does my speedometer read wrong after changing tire sizes?
Your speedometer measures how fast your wheels are rotating, not your actual ground speed. It's calibrated for your factory tire's exact circumference. A larger tire covers more distance per rotation, so your actual speed is higher than what the speedometer shows. A smaller tire has the opposite effect.
What does "revolutions per mile" mean and why does it matter?
It's how many times your tire fully rotates to cover one mile. This number directly determines your speedometer and odometer accuracy, and it's also used to recalibrate your vehicle's computer (ECU) after installing different-sized tires, especially important for trucks with aftermarket gearing.
How much can I change my tire size before it's a problem?
Most manufacturers and tire professionals recommend staying within about 3% of your original overall tire diameter. Going beyond that can throw off your speedometer, ABS calibration, and in some cases affect transmission shift points on newer vehicles.
Can I run different tire sizes on the front and rear?
On most passenger vehicles, no — mismatched front/rear sizes can damage all-wheel-drive systems and affect handling. Staggered fitments (different front/rear sizes) are common on rear-wheel-drive performance cars, but always check your manufacturer's specifications first.
Does a bigger tire really improve off-road performance?
Larger diameter tires increase ground clearance and can improve traction with a larger contact patch, but they also add rotational mass (reducing acceleration and fuel economy) and may require gear ratio changes to maintain performance. It's a genuine trade-off, not a free upgrade.
What's the difference between overall diameter and rim diameter?
Rim diameter is just the wheel size (the "R" number, e.g. 17 inches). Overall diameter is the full height of the mounted tire — rim diameter plus twice the sidewall height. Two tires can share the same rim size but have very different overall diameters depending on width and aspect ratio.
Will a tire size calculator work for both metric and flotation sizes?
Yes. Metric sizes (like 225/65R17) and flotation sizes (like 31x10.5R15) use different notations but describe the same physical dimensions. Flotation sizes state overall diameter and width directly in inches, which is why they're common on trucks and off-road tires where quick sizing matters.
Should I recalibrate my speedometer after a tire size change?
If the size difference is more than about 3%, yes. On many modern vehicles this can be done through the onboard computer or by a shop with the right diagnostic tools. Ignoring a significant size change means your speedometer, odometer, and fuel economy readout will all be inaccurate.
