How do I read a tire size code like 225/65R17?

A tire size like 225/65R17 means: 225 = section width in millimeters (how wide the tire is); 65 = aspect ratio (sidewall height as a percentage of width — 65% of 225mm = 146.25mm); R = radial construction; 17 = rim/wheel diameter in inches. The overall tire diameter = rim diameter + 2 × sidewall height = 17 + 2×(146.25/25.4) = 17 + 11.52 = 28.52 inches.

What does the aspect ratio on a tire mean?

The aspect ratio is the second number in a tire size code, expressed as a percentage. It indicates the tire's sidewall height as a percentage of its section width. A 225/45R17 tire has a sidewall height of 45% × 225mm = 101.25mm. Lower aspect ratios (35, 40, 45) mean shorter sidewalls — typical on sporty and performance vehicles for better handling. Higher aspect ratios (65, 70, 75) mean taller sidewalls — common on trucks and SUVs for a smoother ride.

How do I calculate tire diameter?

Tire diameter = Rim diameter (inches) + 2 × Sidewall height (inches). Sidewall height (mm) = Width × (Aspect ratio / 100). Convert to inches by dividing by 25.4. Example: 225/65R17: Sidewall = 225 × 0.65 = 146.25mm = 5.758 inches. Diameter = 17 + 2×5.758 = 28.52 inches.

How does changing tire size affect the speedometer?

Your speedometer is calibrated to your original tire's circumference. If you install tires with a larger overall diameter, your actual speed will be higher than what the speedometer shows (speedometer reads low). Smaller tires make the speedometer read high. Speedometer error % = ((new diameter - old diameter) / old diameter) × 100. A 3% change means your speedometer is off by about 3 mph at 60 mph.

What are revolutions per mile for a tire?

Revolutions per mile = 63,360 (inches in a mile) / (π × tire diameter in inches). Example: a 28.52 inch diameter tire: rev/mile = 63,360 / (3.14159 × 28.52) = 63,360 / 89.61 = 707 rev/mile. Larger tires = fewer revolutions per mile. This matters for gear ratio calculations, ABS calibration, and odometer accuracy.

What is the maximum allowable tire size difference?

Most vehicle manufacturers and tire experts recommend staying within 3% of the original tire's overall diameter when changing tire sizes. Exceeding 3% can affect speedometer accuracy, ABS calibration, stability control, odometer readings, and potentially cause clearance issues. Always verify wheel well clearance and check for rubbing when turning and over bumps when installing larger-than-stock tires.

What is plus-sizing in tires?

Plus-sizing means fitting a larger diameter wheel (rim) while using a lower-profile tire with a smaller sidewall, maintaining approximately the same overall tire diameter. Plus 1 = increase rim by 1 inch (e.g., 17 to 18); Plus 2 = increase by 2 inches. This improves handling precision (shorter sidewall flex) and visual appearance, while keeping speedometer accurate. The tire width often increases slightly with each plus size.

What do P, LT, ST, and T mean at the start of tire sizes?

P = Passenger car (most common; designed for cars, SUVs, crossovers, minivans, and light-duty pickups). LT = Light Truck (built for heavier loads; required for vehicles with GVWR over 8,500 lbs). ST = Special Trailer (for trailers and towable equipment only — never use on a vehicle). T = Temporary (for spare tires only; not for extended use). Euro-metric tires have no letter prefix.

What is tire section width vs overall width?

Section width is the width of the tire from sidewall to sidewall measured at the widest point, without vehicle weight on the tire. The first number in a metric tire code (e.g., 225 in 225/65R17) is the section width in millimeters. The overall mounted width may be slightly wider than the section width due to the mounting flange. This is different from tread width, which is only the contact patch with the road.

How do I convert tire sizes from metric to inches?

To find the full tire dimensions in inches from a metric code (e.g., 225/65R17): Width = 225mm ÷ 25.4 = 8.86 inches. Sidewall = 225 × 0.65 = 146.25mm ÷ 25.4 = 5.76 inches. Diameter = 17 + (2 × 5.76) = 28.52 inches. Circumference = π × 28.52 = 89.6 inches. Revolutions/mile = 63,360 ÷ 89.6 = 707.

225/65R17 · Diameter · Speedometer

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Original Tire 225/65R17

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How to Read a Tire Size

Decoding the Tire Size Code — 225/65R17

Every metric tire has a standardized code on the sidewall. Here's what each number means.

225

Section Width — 225 mm wide, sidewall to sidewall. Divided by 25.4 for inches.

/65

Aspect Ratio — Sidewall height = 65% of 225 mm = 146.25 mm. Lower = sportier.

R

Radial construction. Nearly all modern passenger tires are radial (R).

17

Rim Diameter — 17-inch wheel required. This is always in inches.

Common Sizes

Popular Tire Sizes — Dimensions at a Glance

Click any size to load it into the calculator above.

Tire Size	Diameter (in)	Width (in)	Sidewall (in)	Circumference (in)	Rev/Mile	Common Vehicles

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Stay Within 3% of Original Diameter When Upsizing

Exceeding 3% difference in overall diameter affects speedometer accuracy, ABS and traction control calibration, odometer readings, and clearance. A 3% larger diameter tire at an indicated 60 mph means you're actually going ~61.8 mph. Always check for rubbing at full lock and over bumps. Most wheel well clearances allow 1–2% diameter change without modification.

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Low Profile vs High Profile

Low profile (aspect ratio 35–50): Short sidewall. Better handling precision, less sidewall flex through corners. Less comfortable ride — shorter sidewall absorbs fewer road imperfections. Common on sports cars and performance vehicles.

High profile (aspect ratio 60–80): Tall sidewall. More cushioning over rough roads, better for off-road. More flex = slightly less steering precision. Common on trucks, SUVs, and economy vehicles. Also better for pothole resistance — less rim damage risk.

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Plus-Sizing Explained

Plus-sizing means fitting a larger wheel (rim) with a lower-profile tire, maintaining approximately the same overall diameter. Plus 1 = increase rim by 1 inch with reduced aspect ratio. The result: improved steering response (shorter sidewall = less flex), better braking, and improved appearance.

The key rule: keep overall diameter within 3% of stock. Going from 225/65R17 to 235/55R18 (+1 rim) keeps diameter virtually identical (28.5 vs 28.2 in) while improving the wheel-to-tire look and handling feel.

How to Calculate Tire Size Dimensions

All tire dimensions can be calculated from the three numbers in a metric tire code: section width (mm), aspect ratio (%), and rim diameter (inches).

Complete Tire Dimension Formulas

Given tire code: W/A R (e.g. 225/65R17) W = Section width in mm A = Aspect ratio (%) R = Rim diameter in inches Sidewall height (mm) = W × A / 100 Sidewall height (in) = W × A / 100 / 25.4 Overall diameter (in) = R + 2 × (W × A / 100 / 25.4) Overall diameter (mm) = R × 25.4 + 2 × (W × A / 100) Circumference (in) = π × Diameter (in) Revolutions/mile = 63,360 / Circumference (in) Revolutions/km = 39,370 / Circumference (in) Example: 225/65R17 Sidewall = 225 × 65 / 100 = 146.25 mm = 5.758 in Diameter = 17 + 2 × 5.758 = 28.516 in = 724.3 mm Circumference = π × 28.516 = 89.60 in = 227.6 cm Rev/mile = 63,360 / 89.60 = 707.1 rev/mile

Speedometer Error Formula

Speedometer error % = ((New diameter - Old diameter) / Old diameter) × 100 Actual speed = Indicated speed × (New diameter / Old diameter) If new tire is LARGER: speedometer reads LOW (actual speed > indicated speed) If new tire is SMALLER: speedometer reads HIGH (actual speed < indicated speed) Example: Original 28.52 in → New 29.50 in (+3.44%) At indicated 60 mph: actual = 60 × (29.50/28.52) = 62.1 mph Odometer: records fewer miles than actually driven (reads slow)

Load Index and Speed Rating

After the tire code, you'll often see a number and letter (e.g., 95H). The number is the load index — 95 means each tire can support up to 1,521 lbs (690 kg). The letter is the speed rating — H means approved for sustained speeds up to 130 mph (210 km/h). Common speed ratings: T (118 mph), H (130 mph), V (149 mph), W (168 mph), Y (186 mph).

Frequently Asked Questions

225 = section width in mm (sidewall to sidewall). 65 = aspect ratio (sidewall height = 65% of 225mm = 146.25mm). R = radial construction. 17 = rim diameter in inches. Overall diameter = 17 + 2×(146.25/25.4) = 28.52 inches. The tire code is always on the sidewall — look for the P225/65R17 or similar format.

Aspect ratio = sidewall height as a percentage of section width. A 225/45R17 tire has a sidewall = 45% × 225mm = 101.25mm. Lower aspect ratios (35–50): shorter sidewalls, sportier handling, firmer ride. Higher aspect ratios (60–80): taller sidewalls, better cushioning, smoother ride. The sidewall height directly affects overall tire diameter and ride comfort.

Your speedometer is calibrated for your stock tire diameter. Larger diameter tire → speedometer reads low (going faster than shown). Smaller diameter → reads high (going slower than shown). Speedometer error % = ((new - old) / old) × 100. Example: 3% larger tire: at indicated 60 mph, you're actually going 61.8 mph. Use our Compare tab to see the exact error for any two tire sizes.

Revolutions per mile = 63,360 inches/mile ÷ tire circumference (inches). Example: 28.52 inch diameter tire: circumference = π × 28.52 = 89.6 in; rev/mile = 63,360 / 89.6 = 707. Larger tires = fewer rev/mile. Matters for odometer accuracy, ABS calibration, and gear ratio calculations. Changing tire size changes rev/mile and thus affects all distance-based systems.

Most experts recommend staying within 3% of original overall diameter. Exceeding 3% risks: speedometer error greater than 3 mph at 60 mph, ABS/traction control miscalibration, potential clearance issues (rubbing), and strained CV joints on FWD vehicles. Always physically verify clearance at full steering lock and over bumps. Our Compare tab shows the percentage difference automatically.

Plus-sizing = larger rim + lower-profile tire, maintaining same overall diameter. Plus 1: increase rim 1 inch, reduce aspect ratio to compensate. Benefits: sharper steering (less sidewall flex), better braking, improved aesthetics. The rule: keep overall diameter within 3% of stock. Example: 225/65R17 → 235/55R18 (+1) → diameter changes from 28.52 to 28.19 inches (−1.16%) ✓

P = Passenger car (cars, SUVs, crossovers, minivans, light pickups). LT = Light Truck (heavier loads; GVWR over 8,500 lbs). ST = Special Trailer (trailers only — never use on a vehicle). T = Temporary (spare tires only). Euro-metric tires have no prefix. LT tires have a higher load rating and require different inflation pressure than P-metric tires of the same size.

Diameter (in) = Rim (in) + 2 × Sidewall (in). Sidewall (mm) = Width × Aspect/100. Convert to inches: ÷25.4. Example 225/65R17: Sidewall = 225×0.65 = 146.25mm = 5.758in. Diameter = 17 + (2×5.758) = 28.516 in. Or use our Tire Decoder above — just select width, aspect, and rim from the dropdowns.

Section width = tire width at widest point, sidewall to sidewall, unmounted. It's the first number in the code (e.g., 225 in 225/65R17 = 225mm). Overall mounted width may differ slightly. Tread width is narrower — only the flat contact patch portion. When upgrading width, verify it fits between fender liners, especially at full steering lock. Generally safe to go one width step (10mm) wider than stock.

From 225/65R17: Width = 225÷25.4 = 8.86 in. Sidewall = 225×0.65÷25.4 = 5.76 in. Diameter = 17 + (2×5.76) = 28.52 in. Circumference = π×28.52 = 89.6 in. Rev/mile = 63,360÷89.6 = 707. Our calculator shows all these automatically — just select your tire size dimensions in the dropdowns above.

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