Decode tire size numbers, compare tire sizes, and calculate speedometer differences. Understand tire specifications instantly.
Tire size codes follow a standardized format that tells you everything about the tire's dimensions:
Beyond the size code, tires have other important markings:
Plus sizing is upgrading to larger diameter wheels while maintaining the same overall tire diameter. This typically means wider tires with lower aspect ratios. For example, going from 225/55R17 to 235/50R18.
When changing tire sizes, stay within these guidelines:
| Original Size | Plus 1 | Plus 2 | Diameter Change |
|---|---|---|---|
| 205/55R16 | 225/50R17 | 235/45R18 | +0.1 to +0.3" |
| 225/60R17 | 235/55R18 | 245/50R19 | +0.2 to +0.4" |
| 235/65R17 | 245/60R18 | 255/55R19 | +0.1 to +0.5" |
| 265/70R17 | 275/65R18 | 285/60R20 | +0.3 to +0.8" |
A tire code like 225/55R17 encodes three key measurements: Section Width (225mm) / Aspect Ratio (55%) / Rim Diameter (17 inches). Here's what each number means and how changing tire size affects your vehicle:
| Code Component | What It Means | Effect of Going Larger |
|---|---|---|
| Width (e.g., 225) | Tread width in mm | Better grip, higher rolling resistance, more road noise |
| Aspect Ratio (e.g., 55) | Sidewall height as % of width = 55% × 225 = 124mm sidewall | More cushioning, less sporty response |
| Rim Diameter (e.g., 17) | Wheel diameter in inches | Lower profile look, but must keep overall diameter the same |
| Diameter Change | Speedometer Effect | Odometer Effect | Acceptable? |
|---|---|---|---|
| +3% larger | Reads ~3% low (actual speed higher) | Under-counts miles by 3% | OK |
| -3% smaller | Reads ~3% high (could trigger speed cameras) | Over-counts miles × warranty concern | Marginal |
| ×5%+ change | Significant speedometer error | ABS/traction control miscalibration | Not recommended |
Your tire size is molded into the sidewall of your current tires. Look for a sequence like "225/55R17 94H" on the outer sidewall. You can also find the recommended tire size in your vehicle's owner's manual, driver's door jamb sticker, or fuel filler door. The door jamb sticker shows the original equipment tire sizes and recommended pressures.
Yes, but with limitations. The new tire's overall diameter should be within 3% of the original. Larger differences will cause speedometer error, affect gear ratios, and potentially cause clearance issues. Always maintain or exceed the load and speed ratings. Consult a tire professional before changing sizes, and consider how it affects handling, fuel economy, and warranty coverage.
Incorrect tire diameter affects speedometer accuracy because the vehicle's computer calculates speed based on wheel rotations. Larger diameter tires mean fewer rotations per mile, so your speedometer will read slower than actual speed. Conversely, smaller tires make it read faster. A 3% difference means your speedometer shows 60 MPH when you're actually going 58.2 or 61.8 MPH. This affects speed enforcement tickets and odometer accuracy.
Aspect ratio determines sidewall height. Lower aspect ratios (45, 40, 35) provide: better handling and steering response, improved high-speed stability, sportier appearance, but harsher ride quality and greater wheel damage risk. Higher aspect ratios (65, 70, 75) offer: more comfortable ride, better cushioning over bumps, increased off-road capability, lower cost, but reduced handling precision and more sidewall flex.
For most vehicles, yes. Mismatched tire sizes can damage all-wheel drive (AWD) systems, cause handling imbalances, trigger traction control errors, and create uneven wear. Some high-performance and sports cars use staggered fitments (wider rear tires) from the factory. In emergencies, temporarily using a different-sized spare is acceptable for short distances. Always check your owner's manual for specific requirements, especially for AWD vehicles which are particularly sensitive to tire diameter differences.
Wider tires provide: larger contact patch for better dry traction, improved cornering grip, enhanced braking performance, and more aggressive appearance. However, they also cause: increased rolling resistance (worse fuel economy), higher cost, greater weight, potential for hydroplaning on wet roads, and possible rubbing issues if too wide. The optimal width balances performance needs with practical considerations. Typically, staying within 10-20mm of the original width is safe.
These letters indicate tire construction: R (Radial) - most common, cords run perpendicular to direction of travel, better fuel economy and comfort, longer tread life. D (Diagonal/Bias) - older technology, cords crisscross at angles, mainly used on trailers and some trucks, stiffer sidewalls. B (Belted-Bias) - combination design, rarely used today. Over 98% of modern passenger vehicle tires are radial construction due to superior performance, longevity, and fuel efficiency.