Free Tire Size & Wheel Offset Calculator
Compare OEM vs new wheel fitment: calculate inner strut clearance change, outer fender poke, speedometer calibration error, and rolling diameter.
🛞 Wheel & Tire Dimensions
OEM / Existing Wheel & Tire
New / Aftermarket Wheel & Tire
📊 Clearance & Speedometer Changes
Recommended Tools & Equipment
Tested hardware and components for high reliability
Wheel Offset (ET) & Clearance Mechanics
Wheel offset (measured in millimeters as ET, from the German Einpresstiefe) is the distance from the wheel's true geometric centerline to the mounting face touching the brake rotor hub.
Positive vs Negative Offset
Most modern front-wheel-drive and AWD passenger cars use positive offset (+35 to +50mm), tucking the wheel inward. Decreasing offset (e.g. from +45 to +35) pushes the wheel outward towards the fender by 10mm.
The 3% Rolling Diameter Rule
When changing wheel and tire sizes, automotive manufacturers strongly advise keeping overall rolling diameter within ±3% of factory spec. Deviations greater than 3% cause transmission shift timing problems, speedometer inaccuracies, and ABS/traction control errors.
Frequently Asked Questions
What does wheel offset (ET) mean?
ET is the millimeter offset from the wheel's centerline to its hub mounting surface. A lower ET pushes the wheel further out toward the fender; a higher ET pulls the wheel deeper inside the wheel well.
What happens if a new wheel has too much inner clearance loss?
If the wheel extends too far inward, the inner rim lip or tire sidewall will rub against the suspension coilover strut, sway bar, or inner wheel liner, causing dangerous tire blowouts.
How do I calculate tire sidewall height?
Sidewall height equals tire width multiplied by the aspect ratio percentage. For a 225/45R17 tire: $225 \text{ mm} \times 0.45 = 101.25 \text{ mm}$.