Plus Sizing Tires and Wheels: Size Comparisons and Fitment Guide

Plus sizing means installing larger diameter wheels with lower profile tires to keep the overall tire diameter close to the original. For example, replacing 205/55R16 with 225/45R17 or 225/40R18. The tire sidewall numbers tell you width, aspect ratio, and rim diameter in a standard code like P225/45R17 91V.

What Do the Numbers on a Tire Sidewall Mean?

A marking like P225/45R17 91V breaks down as:

• P – passenger tire (optional letter).
• 225 – section width in millimeters from sidewall to sidewall (not tread width).
• 45 – aspect ratio: sidewall height is 45% of the section width.
• R – radial construction.
• 17 – rim diameter in inches.
• 91 – load index (maximum load per tire; 91 = 615 kg / 1356 lbs).
• V – speed rating (max speed 240 km/h / 149 mph).

Sidewall height in millimeters = width × aspect ratio ÷ 100. For 225/45R17: 225 × 0.45 = 101.25 mm (about 4 inches). Overall tire diameter = rim diameter in mm + 2 × sidewall height. 17 inches = 431.8 mm, so overall diameter = 431.8 + 2×101.25 = 634.3 mm (24.97 inches). Circumference = π × diameter ≈ 1992 mm. These are nominal calculated dimensions; actual dimensions vary slightly by manufacturer and tire model.

How Plus Sizing Changes Tire Dimensions

When you go from a 16-inch to a 17-inch wheel (plus-one sizing) or 18-inch (plus-two sizing), you must reduce the aspect ratio to keep overall diameter similar. For example, if original is 205/55R16:

• Sidewall height = 205×0.55 = 112.75 mm. Rim = 406.4 mm. Overall diameter = 406.4 + 225.5 = 631.9 mm.
• Plus-one: 225/45R17. Sidewall = 101.25 mm. Rim = 431.8 mm. Overall diameter = 431.8 + 202.5 = 634.3 mm. Difference ≈ +0.4%.
• Plus-two: 225/40R18. Sidewall = 90 mm. Rim = 457.2 mm. Overall diameter = 457.2 + 180 = 637.2 mm. Difference ≈ +0.8%.

A diameter difference under 3% is generally acceptable for speedometer and gearing. Larger differences affect speedometer readings, ABS calibration, and can cause tire rubbing.

What Is Approved Rim Width and Why Does It Matter?

Every tire size has a range of approved rim widths. For 225/45R17, typical approved rim width is 7.0–8.5 inches, with a measuring rim width of 7.5 inches. Installing a tire on a rim outside this range can distort the tire shape, reduce handling, and cause uneven wear. Wheel width also affects tire clearance: a wider wheel with a different offset may push the tire closer to suspension or fender.

Plus Size Tire Chart for Common Original Sizes

Below are common original sizes and their plus-one and plus-two alternatives with similar overall diameter. Always verify with a tire size calculator and check vehicle clearance.

Differences are approximate; a tire size calculator gives exact numbers.

How to Check Tire Clearance Before Plus Sizing

1. Measure current clearance between tire and fender, suspension, and brake components at full steering lock and suspension compression.
2. Compare overall diameter, section width, and wheel offset of the new combination.
3. Check that the new wheel clears brake calipers (larger diameter wheels often required for big brake kits).
4. Consider wheel offset: lower offset pushes wheel outward, increasing risk of rubbing.
5. Test fit before final purchase if possible.

Speedometer Difference with Plus Sizing

Speedometer error depends on the change in overall tire diameter. If new diameter is larger, speedometer reads slower than actual speed. If smaller, it reads faster. For a 1% diameter increase, the speedometer underestimates speed by about 1%. Example: original diameter 632 mm, new diameter 638 mm = +0.95% difference. At an indicated 60 mph, actual speed ≈ 60.6 mph. Most vehicles have some built-in speedometer error, but keeping diameter within 3% is advised.

Plus Sizing Tire Guide: Step by Step

1. Find your original tire size on the sidewall or door placard.
2. Calculate original overall diameter using formula: (width × aspect ratio × 2 ÷ 25.4) + rim diameter in inches.
3. Choose a new rim diameter 1 or 2 inches larger.
4. Select a tire width that fits the new rim width and vehicle clearance (often 10–20 mm wider than original).
5. Pick an aspect ratio that yields overall diameter within ±3% of original.
6. Verify load index and speed rating meet or exceed original.
7. Check approved rim width range for the new tire size.
8. Confirm wheel bolt pattern, center bore, and offset match your vehicle.

Plenty of online plus sizing calculators automate these steps.

Common Mistakes in Plus Sizing

• Choosing a tire too wide for the wheel well, causing rubbing on suspension or fenders.
• Installing a tire on a rim outside the approved width range.
• Ignoring load index: lower profile tires often have same or higher load index, but must be checked.
• Not considering wheel offset: even correct tire size may rub with wrong offset.
• Assuming all low-profile tires fit all cars of the same size; actual dimensions vary by brand.

Always consult a professional fitment guide or your vehicle manufacturer before making changes.