Why Use a Plus-Size Wheel Fitment Calculator?
Plus sizing is the industry-standard method for upgrading to larger, sportier alloy wheels while retaining original vehicle suspension geometry and rolling radius.
If you mount larger wheels without decreasing tire sidewall height proportionally, the overall tire diameter grows too large—causing speedometer errors, rubbing against fenders, and sluggish acceleration.
Calculation Flow & Mathematical Formulas
Plus sizing follows standard rim diameter additions paired with lower aspect ratios:
Inputs & Parameters
| Parameter | Unit | Description |
|---|---|---|
| OEM Width ((W_{oem})) | mm | Factory tire section width |
| OEM Aspect ((AR_{oem})) | % | Factory aspect ratio |
| OEM Rim ((R_{oem})) | inches | Factory wheel diameter |
| Plus Tier ((K)) | inches | Plus 1 (+1”), Plus 2 (+2”), Plus 3 (+3”) |
Step-by-Step Formulas
1. OEM Overall Diameter ((D_{oem}))
[ H_{oem_sw} = \frac{W_{oem} \times \frac{AR_{oem}}{100}}{25.4} ] [ D_{oem} = R_{oem} + (2 \times H_{oem_sw}) \quad \text{(in inches)} ]
2. Plus-Sized Rim & Overall Diameter ((D_{plus}))
[ R_{plus} = R_{oem} + K ] [ H_{plus_sw} = \frac{W_{new} \times \frac{AR_{new}}{100}}{25.4} ] [ D_{plus} = R_{plus} + (2 \times H_{plus_sw}) \quad \text{(in inches)} ]
3. Compliance Percentage Variance ((\Delta D_{\%}))
[ \Delta D_{\%} = \left(\frac{D_{plus} - D_{oem}}{D_{oem}}\right) \times 100 ]
Plus-Sizing Matrix Example
| Upgrade Tier | OEM Spec | Proposed Plus Spec | OEM Diam | Plus Diam | Diam Diff (%) | Fitment Status |
|---|---|---|---|---|---|---|
| OEM Baseline | 205/55R16 | — | 24.88” | — | 0.00% | Stock |
| Plus 1 (+1”) | 205/55R16 | 225/45R17 | 24.88” | 24.97” | +0.36% | Optimal Fit |
| Plus 2 (+2”) | 205/55R16 | 225/40R18 | 24.88” | 25.09” | +0.84% | Optimal Fit |
| Plus 3 (+3”) | 205/55R16 | 235/35R19 | 24.88” | 25.48” | +2.41% | Acceptable (< 3%) |
Step-by-Step How-To Guide
- Enter OEM Specs: Read stock tire dimensions off door pillar (e.g.
205/55R16). - Select Plus Upgrade: Choose Plus 1 (+1” wheel), Plus 2 (+2” wheel), or Plus 3 (+3” wheel).
- Select New Width & Profile: Test wider widths (e.g.
225) paired with lower aspect ratios (e.g.45or40). - Check Compliance: Verify green “Optimal Fit” or “Acceptable” status under 3% difference.
Frequently Asked Questions
What is Plus 1 wheel upsizing?
Plus 1 sizing increases wheel rim diameter by 1 inch while reducing sidewall aspect ratio (and usually increasing section width) to keep total outer tire height virtually identical to stock.
What is Plus 2 wheel upsizing?
Plus 2 sizing increases wheel rim diameter by 2 inches (e.g. from 16” to 18”) while using a lower-profile tire to maintain OEM overall diameter within ±3%.
Why is it important to keep overall tire diameter within 3%?
Keeping overall tire height within 3% preserves original gearing ratios, maintains ABS and traction control sensor calibration, and prevents speedometer inaccuracies.
What are the performance benefits of Plus sizing?
Plus sizing increases tire contact patch width and shortens sidewall height, reducing sidewall flex during hard cornering for sharper steering response and improved dry handling.
Does Plus sizing make the ride harsher?
Yes. Shorter sidewalls provide less rubber cushioning, resulting in a firmer, stiffer ride over bumps and potholes.
Can Plus sizing cause rim damage from potholes?
Low profile tires used in Plus 2 and Plus 3 fitments have less sidewall cushion, increasing the likelihood of rim damage if hitting severe potholes at speed.
Are my wheel sizing choices stored online?
No. All calculations run strictly inside your web browser.