Calculator Inputs
Example Data Table
| Stock Width | Stock Offset | New Width | New Offset | Spacer | Expected Result |
|---|---|---|---|---|---|
| 8 in | 35 mm | 9 in | 25 mm | 0 mm | More outward poke and less inner clearance. |
| 7.5 in | 45 mm | 8 in | 40 mm | 5 mm | Noticeable outward stance change. |
| 9 in | 30 mm | 9.5 in | 35 mm | 0 mm | Small outer change with more inner usage. |
Formula Used
Rim width in mm = Rim width in inches × 25.4
Effective offset = New offset − Spacer thickness − Adapter thickness
Outer lip change = (New half width − Effective offset) − (Stock half width − Stock offset)
Inner lip change = (New half width + Effective offset) − (Stock half width + Stock offset)
Inner clearance change = − Inner lip change
Track change per axle = 2 × (Stock offset − Effective offset)
Backspacing = (Half rim width + Offset) ÷ 25.4
Negative camber top inset = Tire diameter × sin(abs(camber angle))
How to Use This Calculator
Enter your current wheel width and offset first.
Add the proposed wheel width and offset.
Enter tire section widths for better shoulder estimates.
Add spacer or adapter thickness per side.
Measure current fender and inner clearance if possible.
Use negative camber when the tire top leans inward.
Press calculate to compare poke, inset, and clearance.
Download the result as CSV or PDF for shop review.
Rim Width and Offset Planning
Wheel fitment starts with two numbers. Rim width describes the barrel size. Offset describes the mounting pad position. Both numbers decide where the wheel sits. A wider rim can move inward, outward, or both. Offset controls that movement more than many buyers expect.
Why Offset Matters
Positive offset pulls the wheel toward the suspension. Lower offset pushes the wheel outward. A spacer also pushes the wheel outward because it lowers effective offset. This calculator compares the stock wheel against a proposed setup. It shows outer poke, inner clearance loss, backspacing, centerline shift, and axle track change.
Clearance and Poke Checks
Inner clearance protects struts, shocks, springs, arms, and brake parts. Outer poke affects fenders, mud guards, and legal coverage. A setup may look aggressive but still rub during steering or compression. Tire width also matters. A tire can bulge past the rim. That is why this tool includes tire section width, not rim width only.
Advanced Setup Notes
Use measured stock data when possible. Factory published sizes are useful, but real wheels vary. Tire shoulder shape also varies by brand. Negative camber can pull the tire top inward. It may help fender clearance, yet it can reduce tire life. Suspension drop can also reduce available space. Check both sides of the vehicle. Production tolerances are not always equal.
Practical Fitment Advice
Treat the result as a planning estimate. Keep a safety margin for movement, heat, tire growth, and alignment changes. For daily driving, avoid setups that leave only a tiny gap. For track use, allow more room because loads are higher. Always test fit before final mounting. Turn the steering from lock to lock. Compress the suspension if possible. Inspect brake caliper clearance too.
Measurement Tips
Measure from solid suspension points. Record current gaps before changing parts. Use the same tire pressure during checks. Recheck after alignment, because toe and camber can slightly change shoulder clearance.
Buying With Confidence
The best wheel choice balances appearance, clearance, handling, and comfort. A small offset change can be safe. A large width change needs more review. Compare multiple setups before ordering. Save the result as a CSV or PDF. Share it with a wheel shop when checking fitment.
FAQs
What does rim offset mean?
Offset is the distance from the wheel centerline to the mounting face. Positive offset moves the wheel inward. Lower offset usually moves it outward.
What does rim width change affect?
Wider rims can change both outer poke and inner clearance. The final movement depends on width, offset, spacer thickness, and tire section width.
Does a spacer change offset?
Yes. A spacer lowers effective offset. A 10 mm spacer makes a 35 mm offset behave like a 25 mm offset.
What is backspacing?
Backspacing is the distance from the hub mounting face to the inner rim edge. It helps estimate suspension-side clearance.
Why include tire width?
Tire shoulders can extend past the rim. Tire width helps estimate real outer poke and inner clearance more accurately than rim data alone.
Can this confirm perfect fitment?
No. It gives a strong estimate. Always test fit because tire shape, alignment, brakes, suspension travel, and vehicle tolerances can change clearance.
What does positive outer change mean?
Positive outer change means the wheel or tire moves outward. This can improve stance, but it may increase fender rubbing risk.
What clearance margin is safe?
Daily cars usually need a practical safety margin. More clearance is better for steering, suspension compression, tire growth, and alignment changes.