Will this tire size fit my car? (The 3% rule explained)
Before you install a different tire size — whether you are going wider, changing aspect ratio, or trying a plus size — there is one number that determines whether the swap is safe: the overall diameter difference, expressed as a percentage.
The 3% rule
A common fitment guideline: the new tire's overall diameter should differ from the original by no more than ±3%. This threshold comes from the interaction of diameter with:
- Speedometer accuracy — your speedometer is calibrated to a specific tire diameter. A different diameter means a proportionally different reading.
- ABS and traction control calibration — wheel speed sensors calculate vehicle speed from wheel rotation rate. A different diameter produces a different signal, and at large differences, can confuse stability control systems.
- Odometer accuracy — recorded mileage scales with tire circumference.
The math behind the rule
Overall diameter is computed from the size:
OD_mm = rim_in × 25.4 + 2 × (section_width × aspect_ratio / 100)The percentage difference is:
diameter_diff_pct = (OD_new - OD_stock) / OD_stock × 100A value within ±3% is generally safe. Between 3–5% needs case-by-case review. Above 5% is usually problematic and may cause ABS faults or contact with suspension components.
Physical clearance: the second check
Even within the 3% rule, you must also verify physical fitment in your specific wheel well:
- Full suspension travel — the tire must not contact the fender liner, strut, or control arm at full compression (as in a hard bump) or full steering lock.
- Wider tires and offset — a wider tire needs either a wider rim or a different wheel offset to avoid rubbing on the inner suspension. Increasing section width without changing rim width can cause sidewall bulge that contacts the strut.
- Ground clearance — a larger diameter tire raises the vehicle by half the diameter difference. A smaller tire lowers it.
How to check quickly
- Find your OEM (stock) tire size from the door jamb sticker or owner's manual.
- Decode both the stock size and the candidate size with the TreadSize compare tool.
- Read the diameter difference percentage. Green (≤3%) = generally safe. Amber (3–5%) = verify carefully. Red (>5%) = proceed with caution.
- Have a tire shop verify physical clearance at full travel before you install.
When the 3% rule doesn't apply
- Identical size, different brand — same OD, no math needed.
- Winter/snow tires on a dedicated set of rims — many owners fit slightly narrower, taller winter tires specifically for rolling resistance and snow traction. Recalibrate expectations for speedometer error.
- Lifted trucks with a speedometer recalibration — if you have an aftermarket programmer or differential re-gear to correct the speedo, you can exceed 3% — but this requires proper calibration of all safety systems.
Resources for further verification
For detailed fitment data by vehicle (fender liner clearance, offset specs, hub bore), caroutlay.com maintains vehicle-specific cost and spec data that can help you cross-reference OEM specs. Always confirm the final decision with a qualified tire shop before purchase.
Diameter percentages on TreadSize are computed using standard ETRTO formulas. The 3% guideline is widely referenced by tire manufacturers; consult your vehicle's owner manual and a qualified tire professional for vehicle-specific advice.