Tire Size Calculator

Decode tire size markings and compare different tire sizes for your vehicle. This tool processes all data locally in your browser. No information is ever sent to any server. Completely free, no registration required.

How to Use the Tire Size Calculator

1. Enter your input values above
2. Results update automatically
3. Copy or download the output

What is a Tire Size Calculator?

A Tire Size Calculator decodes the cryptic numbers on your tire sidewall (e.g., P225/65R17 102H) and computes all key dimensions: overall diameter, sidewall height, section width, circumference, revolutions per mile, and speedometer error when changing tire sizes. Understanding tire dimensions is essential when: buying replacement tires, upgrading to larger wheels, correcting speedometer readings, checking clearance for wider tires, or comparing tire options. A wrong tire size can affect safety, fuel economy, and speedometer accuracy.

How Does It Work?

Enter your tire size code (e.g., 225/65R17). The calculator parses: Width (225mm), Aspect Ratio (65% of width = sidewall height), Construction (R = Radial), Wheel Diameter (17 inches). It computes: Sidewall Height = Width × Aspect Ratio ÷ 100, Overall Diameter = Wheel + 2×Sidewall, Circumference = π × Diameter, Revs per Mile = 63,360 ÷ Circumference. The comparison mode shows the difference between two tire sizes: diameter change, speedometer error at 60mph, and clearance change.

Formula

Tire Size Decoding: P225/65R17 102H\n\nP = P-metric (passenger) | LT = Light Truck\n225 = Section Width in millimeters\n65 = Aspect Ratio % (sidewall ÷ width × 100)\nR = Radial construction\n17 = Rim Diameter in inches\n102 = Load Index (1,874 lbs max load)\nH = Speed Rating (130 mph max)\n\nDimensions:\nSidewall Height = Width × Aspect Ratio ÷ 100 (mm)\nOverall Diameter = Rim Diameter × 25.4 + 2 × Sidewall Height (mm)\nOverall Diameter (in) = mm ÷ 25.4\nCircumference = π × Diameter (mm)\nRevs per Mile = 1,609,344 ÷ Circumference (mm)\n\nSpeedometer Error = (New Diameter − Old Diameter) ÷ Old Diameter × 100%\nAt 60 mph: Actual Speed = 60 × (1 + Error%)

Who Uses This Tool?

• Car owners buying replacement tires and comparing sizes
• Enthusiasts upgrading to larger diameter wheels (plus-sizing)
• Off-roaders fitting larger tires and calculating speedometer correction
• Mechanics verifying tire fitment before ordering
• Anyone seeing tire numbers and wanting to understand what they mean

Pro Tips

• The 'plus one' rule: when increasing wheel diameter by 1 inch, decrease the aspect ratio by 10 and keep width similar to maintain the same overall diameter
• Speedometer error: if new tires are 5% larger, at indicated 60 mph you're actually going 63 mph. At 70 mph, you're going 73.5 — enough for a speeding ticket
• Wider tires improve grip but reduce fuel economy (more rolling resistance). Each 10mm width adds about 1-2% rolling resistance
• Tire pressure affects diameter slightly — under-inflated tires have a smaller effective radius and will show slightly high on the speedometer

Frequently Asked Questions about Tire Size Calculator

Can I fit bigger tires on my car?

Check: (1) overall diameter stays within 3% of original (prevents speedometer/suspension/ABS issues), (2) width fits within wheel well without rubbing at full steering lock and suspension compression, (3) tires fit your existing rims (check rim width range for tire width).

What's the best tire speed rating?

H (130 mph) and V (149 mph) are common for passenger cars and more than sufficient for legal driving in any country. Higher ratings (W, Y) are for performance cars. Never install tires with a LOWER speed rating than the vehicle manufacturer specifies.

Free online Tire Size Calculator — no signup, 100% client-side processing. All data stays in your browser.