Plus sizing explained: +1, +2, and the diameter rule
Plus sizing — going up one or two rim-inch increments while keeping the overall tire diameter constant — is popular for better handling, a wider tire footprint, and a sportier look. But the math matters: get it wrong and you break the speedometer, stress the ABS, or rub the fender liner.
What is plus sizing?
A plus-one (+1) fitment means you go from, say, a 17-inch rim to an 18-inch rim. A plus-two (+2) means two extra rim inches. The compensating adjustment: drop the aspect ratio (sidewall height percentage) so the overall outside diameter stays within ~±3% of the original.
The classic rule of thumb
Per +1 inch of rim diameter:
- Drop aspect ratio by ~10 points
- Optionally increase section width by 10–20 mmExample: your stock size is 215/55R17.
Plus-1 target: 225/45R18 or 235/40R18
Plus-2 target: 235/35R19 or 245/35R19The TreadSize plus sizing calculator automates this for any base size, checking each candidate against the 3% rule.
Why keep the overall diameter the same?
Your speedometer, odometer, ABS, and traction control all assume the stock tire's rolling circumference. Change that circumference too much and:
- Speedo reads off (proportionally to the diameter change).
- ABS wheel-speed sensors see a different signal — large deltas can trigger fault codes.
- Odometer recorded mileage shifts.
The 3% diameter tolerance is the widely-accepted window where these systems remain accurate enough for safe street use.
The trade-offs
| What changes | Effect |
|---|---|
| Shorter sidewall | Stiffer lateral response (handling gain), harsher ride over potholes |
| Wider footprint | More dry grip, potentially worse wet/snow performance |
| Heavier wheel+tire assembly | Higher unsprung mass — can hurt ride quality and fuel economy |
| More rim exposure | Curb rash risk increases |
| Cost | Larger rims and lower-profile tires are typically more expensive per tire |
Checks before you commit
- Diameter delta ≤3% — use the calculator.
- Load index ≥ OEM — the new tire must carry at least the same load as the original.
- Speed rating ≥ OEM — do not go down a speed rating class.
- Physical clearance at full travel — have a shop verify wheel-well clearance with the vehicle loaded and suspension at full compression before you drive.
- Offset and hub bore — a wider tire on a wider rim changes the effective offset. Make sure the hub bore matches and there is no inner contact with the strut.
Try the calculator
Enter your stock size at TreadSize Plus Sizing to see safe +1 and +2 candidates — each with the exact diameter delta shown.
Plus-size candidates are computed using standard ETRTO diameter formulas. Always verify load index, speed rating, and physical clearance with a qualified tire professional before purchase and installation.