Calculator Inputs
Use car presets for quick examples. Always confirm your real factory wheel, tire, brake, hub, and suspension data before ordering.
Formula Used
The calculator converts wheel width from inches to millimeters. It then compares the inner and outer wheel positions.
Effective offset = new offset - spacer thickness
Inner position = wheel width mm / 2 + offset
Outer position = wheel width mm / 2 - offset
Inner change = new inner position - old inner position
Outer poke = new outer position - old outer position
Tire diameter is calculated as:
rim inches × 25.4 + 2 × tire width × aspect ratio / 100.
Speedometer error compares new tire diameter with the old tire diameter.
How To Use This Calculator
- Select a car preset or choose custom values.
- Enter your current wheel width, offset, and tire size.
- Enter your planned wheel width, offset, spacer, and tire size.
- Add your measured suspension and fender clearance.
- Press the calculate button.
- Review inner clearance, outer poke, speed error, and fitment score.
- Download the CSV or PDF report for records.
Example Data Table
| Vehicle | Current Wheel | New Wheel | Spacer | Expected Result |
|---|---|---|---|---|
| Honda Civic Example | 17x7 ET45 | 18x8.5 ET38 | 0 mm | More outer poke, slightly tighter inner clearance |
| BMW 3 Series Example | 18x8 ET34 | 19x9 ET42 | 5 mm | Effective ET37, moderate inner change |
| Ford F-150 Example | 17x8.5 ET44 | 18x9 ET18 | 0 mm | Large outward stance increase |
Wheel Offset Guide For Better Fitment
What Wheel Offset Means
Wheel offset is the distance from the mounting pad to the wheel centerline. A positive offset moves the wheel inward. A lower offset moves it outward. That outward move is often called poke. Offset affects handling, bearing load, brake space, tire rub, and vehicle stance. A small number can create a big visual change.
Why Car Data Matters
Every car has different limits. Fender shape, suspension design, brake size, hub bore, and tire width all matter. Two cars may use the same wheel size but need different offsets. A preset gives a starting point. Your measured clearance gives the real answer. Always measure both sides of the car. One side can have less room than the other.
Inner Clearance And Outer Poke
Inner clearance shows how close the new wheel moves toward struts, shocks, control arms, or liners. A positive inner change means less room. Outer poke shows how far the wheel moves toward the fender. A positive outer change means the wheel sits farther out. Tires may need more space than wheels because sidewalls bulge.
Spacers And Effective Offset
A spacer lowers the effective offset. For example, a 40 mm offset wheel with a 5 mm spacer acts like 35 mm offset. This gives more outer poke and more inner clearance. It can also increase stress on studs, bearings, and hubs. Use quality hardware and proper torque.
Tire Diameter And Speed Error
Tire size affects ride height and speedometer reading. A taller tire travels farther per rotation. Your actual speed becomes higher than the displayed speed. A shorter tire does the opposite. Many street builds try to stay within about three percent of stock diameter. Larger changes may affect gearing, stability control, and clearance.
Final Fitment Check
Use this calculator before buying wheels. Then confirm with test fitting. Turn the steering fully. Compress the suspension. Check brake caliper space. Review local safety rules. A clean fitment should clear the suspension, fender, brakes, and hub while keeping the tire safely covered and aligned.
FAQs
What is wheel offset?
Wheel offset is the distance between the wheel mounting face and the wheel centerline. Positive offset moves the wheel inward. Lower offset usually pushes the wheel outward.
What does outer poke mean?
Outer poke means how far the new wheel moves toward the fender compared with the current wheel. More poke can improve stance but may cause rubbing.
What does inner clearance mean?
Inner clearance means the space between the inner wheel barrel and nearby suspension parts. Less clearance can cause contact with struts, shocks, arms, or liners.
How does a spacer affect offset?
A spacer reduces effective offset. A 5 mm spacer on a 40 mm offset wheel makes it behave like a 35 mm offset wheel.
Is lower offset always better?
No. Lower offset can improve stance, but too much poke may rub fenders, stress bearings, or make steering feel worse.
Why does tire size matter?
Tire size changes overall diameter, sidewall height, speedometer reading, and clearance. Wider tires can also rub even when wheel clearance looks safe.
Can this calculator confirm brake clearance?
No. Brake clearance depends on spoke design, caliper shape, barrel profile, and templates. Always test fit or use manufacturer brake templates.
Should I measure my car first?
Yes. Measure fender and suspension space before ordering. Real measurements are more reliable than generic fitment claims or forum examples.