Plus-sizing is putting a larger-diameter wheel on your car with a shorter-sidewall tire that keeps the overall diameter the same. Done correctly, it sharpens handling and looks better. Done wrong, it throws off your speedometer, confuses your ABS, and ruins the ride.
The 3% rule
The new tire's overall diameter must be within 3% of the original. More than that and your speedometer reads incorrectly, your ABS and traction control systems get the wrong wheel-speed data, and the suspension may rub the tire under full compression or full lock.
The math, simplified
To find a tire's overall diameter from its size string: (width in mm × aspect ratio ÷ 100) × 2, then add the wheel diameter converted to mm. Divide by 25.4 for inches.
Example: 225/65R17. Sidewall = 225 × 0.65 = 146.25mm. Times 2 = 292.5mm of sidewall total. Plus the wheel: 17 inches × 25.4 = 431.8mm. Total: 724.3mm, or 28.5 inches diameter.
Common plus-size paths
Plus 1: Original 225/65R17 → 235/55R18 (28.2 inches). Within 3%, good.
Plus 2: Original 225/65R17 → 245/45R19 (27.7 inches). Within 3%, but the sidewall is much shorter — ride degrades more.
Plus 3: Original 225/65R17 → 255/35R20 (27.0 inches). At the edge of the 3% rule and possibly outside it. Ride and impact risk are dramatically higher.
What else needs to be right
- Bolt pattern must match your car's (e.g. 5x114.3 mm)
- Wheel offset must be within a few mm of original to avoid rubbing
- Wheel width must be appropriate for the new tire's width
- Load index of the new tire must equal or exceed the original
- Speed rating equal or higher
What to expect
- Sharper steering response
- Slightly heavier wheel and tire assembly — can hurt acceleration and braking by 1-3%
- Harsher ride, especially on rough pavement
- More road noise
- Higher tire replacement cost
Plus-zero: stay the same diameter, go to a wider tire
Replace your 225/65R17 with a 235/65R17 — same wheel size, slightly wider. The diameter is slightly bigger (1%) but within tolerance. Gets you more grip without changing the suspension geometry. Often the most cost-effective performance upgrade.
Plus-sizing is a math problem before it's an aesthetics decision.