How to Read a Tire Size
Every metric tire size follows the same format: Width/AspectRRimDiameter. For example, in 225/45R17:
- 225 - Section width in millimeters (tread face to tread face)
- 45 - Aspect ratio: sidewall height is 45% of 225mm = 101.25mm
- R - Radial construction (virtually all modern passenger tires)
- 17 - Rim diameter in inches
From these three numbers you can derive every other dimension: overall diameter, circumference, revolutions per mile, and more. That is exactly what this calculator does - and it draws both tires to scale so you can see the difference visually.
Quick Tire Size Decode
225/45R17: 225 = tread width in mm, 45 = sidewall height as % of width (225 x 0.45 = 101.25mm), R = radial, 17 = rim diameter in inches. The overall diameter is the rim size (in mm) plus two sidewall heights.What This Calculator Computes
Enter two tire sizes using the quick-enter box (e.g., 225/45R17) or tune each value individually. The calculator instantly shows:
- Overall Diameter - Rim × 25.4 + 2 × sidewall height. This is the most critical measurement for speedometer accuracy.
- Sidewall Height - Width × (Aspect / 100). Taller sidewalls absorb more road shock; shorter sidewalls improve steering response.
- Circumference - π × Diameter. How far the vehicle travels in one wheel rotation.
- Revolutions per Mile - 63,360 inches ÷ Circumference (inches). The number your speedometer is calibrated to.
- Speedometer Impact - The percentage difference in indicated vs. actual speed if you swap from Tire 1 to Tire 2.
All values update instantly as you type - no submit button needed. Every result is shareable via URL, so you can post the exact comparison link to a forum or send it to a friend.
Common Tire Size Comparisons
Popular tire size swaps and their diameter differences:
| Original | New Size | Orig. Diameter | New Diameter | Difference | Speedo Error |
|---|---|---|---|---|---|
| 205/55R16 | 225/45R17 | 24.88" | 25.05" | +0.17" | +0.68% |
| 225/45R17 | 235/40R18 | 24.97" | 25.41" | +0.44" | +1.76% |
| 235/65R17 | 265/60R18 | 29.00" | 29.50" | +0.50" | +1.72% |
| 245/40R18 | 245/35R19 | 25.71" | 25.75" | +0.04" | +0.16% |
| 265/70R17 | 285/65R18 | 30.61" | 31.50" | +0.89" | +2.91% |
| 275/55R20 | 295/55R20 | 31.91" | 32.80" | +0.89" | +2.79% |
Speedometer & Odometer Impact
Your speedometer counts wheel revolutions and multiplies by the circumference it was factory-calibrated for. When you change tire diameter, the math no longer holds:
- Larger tire (more circumference) → fewer revolutions per mile → speedometer reads lower than actual speed → you are going faster than the dial shows.
- Smaller tire (less circumference) → more revolutions per mile → speedometer reads higher than actual speed → you are going slower than the dial shows.
The formula for actual speed from indicated speed is:
A 3% or larger speedometer error is considered significant. At highway speeds it can affect cruise control calibration, transmission shift points, ABS, and traction control. In AWD vehicles, large diameter differences between axles can stress differentials and transfer cases.
Speedometer Error With Different Sizes
Plus-Sizing Explained
Plus-sizing (or upsizing) means fitting a larger rim diameter while reducing the aspect ratio to keep the overall tire diameter nearly constant. The standard increments are:
- Plus Zero (+0) - Same rim, wider tire, lower aspect ratio. More grip, same diameter.
- Plus One (+1) - 1-inch larger rim, wider tire, lower aspect ratio. Better handling.
- Plus Two (+2) - 2-inch larger rim, even wider, even lower profile. Maximum performance look.
When plus-sizing, the target is to stay within ±1% of the original diameter. Going over 3% puts stress on wheel speed sensors and can trigger ABS/ESC warning lights. Use the visual diagram in this calculator to see whether your candidate sizes are close enough.
The trade-off: lower-profile tires have stiffer sidewalls (better handling, more road feel) but are more susceptible to damage from potholes. High-profile tires (higher aspect ratio) absorb impacts better but flex more, reducing steering precision.
Plus-Sizing Golden Rule
Frequently Asked Questions
A metric tire size has three numbers: 225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), and 17 is the rim diameter in inches. The "R" means radial construction. So 225/45R17 has a 225mm wide tread, a sidewall that is 45% of 225mm = 101.25mm tall, and fits a 17-inch rim.
Yes. Your speedometer measures wheel revolutions. A larger tire travels farther per revolution, so it takes fewer revolutions to cover a mile - your speedometer reads lower than your actual speed. A smaller tire does the opposite. A difference of more than 3% is generally noticeable and should be corrected if possible. Use the Speedometer Impact result in this calculator to see the exact percentage error.
Generally yes, within limits. Most rim widths support a range of tire widths. For example, a 7.5-inch wide rim can safely run tires from 205mm to 245mm wide. Going too wide risks the tire unseating under hard cornering. The ETRTO standard defines compatible ranges. Check the Rim Width to Tire Size Calculator for a precise answer for your rim width.
A plus-size (or plus-zero, plus-one, plus-two) upgrade means fitting a larger rim diameter while reducing the aspect ratio so the overall tire diameter stays nearly the same. For example, going from 205/55R16 to 225/45R17 (+1) keeps nearly the same diameter while improving handling response (stiffer sidewall) and allowing a larger brake rotor. Use the Plus Size Tire Calculator to find matching options.
As a practical rule, a difference greater than 3% in overall diameter causes noticeable speedometer and odometer error. Differences over 5% can affect traction control, ABS, and AWD systems because the wheel speed sensors detect unequal rotation. For spare tires, most manufacturers allow a temporary spare up to 3% smaller in diameter. For regular fitment, aim to stay within 1–2% of the original size.
Revolutions per mile (rev/mi) is how many times your tire rotates for every mile of travel. A standard 225/45R17 makes about 836 revolutions per mile. Your vehicle's speedometer, odometer, and transmission shift points are calibrated to a specific rev/mi figure. When you change tire size, the rev/mi changes, and all these systems are affected. Smaller tires = more revs/mile = speedometer reads HIGH. Larger tires = fewer revs/mile = speedometer reads LOW.
Wider tires have a larger contact patch, which can improve lateral grip on dry pavement. However, on snow and wet surfaces, a narrower tire actually performs better because it puts more weight per square inch on the contact patch, helping it cut through to the road surface. This is why winter tires are often narrower than summer tires for the same vehicle. Tire compound and tread pattern matter more than width in most conditions.