Inner Tire Clearance Calculator
See how close the tire gets to the strut. Use the calculator below. Results update as you type.
How the Inner Tire Clearance Calculator Works
This calculator works by computing how far the inner rim edge moves when the wheel width or offset changes. A positive change means the inner edge moves toward the strut or control arm.
- Enter the current wheel, for example 8.0 in wide at ET35.
- Enter the new wheel, for example 9.0 in wide at ET18.
- Read the inner edge change: −4.3 mm (−0.17 in), away from the strut.
- Add half of any tire width increase to find the inner sidewall change.
The Formula
The formula is Inner Change = (New Width − Old Width) ÷ 2 × 25.4 + (New Offset − Old Offset).
Inner Change (mm) = (W_new − W_old) × 25.4 ÷ 2 + (ET_new − ET_old) Inner Sidewall Change = Inner Change + (Tire W_new − Tire W_old) ÷ 2Worked example: a 7.0 in ET40 to an 8.0 in ET40 moves the inner edge +12.7 mm (+0.50 in) toward the strut.
Conversion Table
The conversion table shows the inner edge change for 8 wheel swaps. Changes run from −12.0 mm (away) to +12.7 mm (closer).
A wider wheel at the same offset always moves the inner edge closer to the strut. A lower offset moves it away.
| Current Wheel | New Wheel | Inner Change (mm) | Inner Change (in) |
|---|---|---|---|
| 7.5 in ET45 | 8.0 in ET45 | +6.3 | +0.25 |
| 8.0 in ET45 | 8.5 in ET45 | +6.3 | +0.25 |
| 8.0 in ET45 | 8.0 in ET35 | -10.0 | -0.39 |
| 8.0 in ET35 | 9.0 in ET18 | -4.3 | -0.17 |
| 7.0 in ET40 | 8.0 in ET40 | +12.7 | +0.50 |
| 8.5 in ET18 | 9.0 in ET0 | -11.7 | -0.46 |
| 9.0 in ET0 | 9.0 in ET-12 | -12.0 | -0.47 |
| 7.5 in ET50 | 8.5 in ET35 | -2.3 | -0.09 |
What Is the Inner Tire Clearance Calculator?
The Inner Tire Clearance Calculator is a tool that predicts the change in space between the inner side of the wheel and tire and the suspension. The inner edge is the tightest spot on many cars: the strut tube, spring perch and upper control arm sit there.
Inner clearance problems cause rubbing at full steering lock and sidewall damage on the strut. The calculator shows the change before you mount the wheel.
Common Uses
This calculator has 4 uses: strut clearance, spring-perch checks, wider-wheel planning and spacer decisions.
- Check strut clearance: See if a wider wheel hits the strut.
- Check spring perches: Measure change near coilovers.
- Plan wider wheels: Pick an offset that keeps inner space.
- Decide on spacers: Know when a spacer is needed.