Wheel Fit Calculator

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Wheel Fitment & Offset Calculator

Calculate clearance and poke when changing wheel specs

Current Wheel Setup

New Wheel Setup

Inner Clearance –
Outer Position (Poke) –

How Wheel Fitment Calculations Work

Upgrading your wheels involves more than just picking a design you like. To ensure your new rims don't rub against the suspension components (inner clearance) or stick out too far from the fender (poke), you must calculate the change in physical positioning based on Width and Offset (ET).

Key Definitions

  • Wheel Width: Measured from bead seat to bead seat. Changing width affects both sides of the wheel equally relative to the centerline.
  • Offset (ET): The distance from the wheel's mounting surface to its true centerline. A higher positive offset moves the wheel further inward toward the car's body.
  • Inner Clearance: The distance between the back of the wheel and the suspension strut. If this number decreases too much, you may need spacers.
  • Poke: How far the outer edge of the wheel sits relative to the hub. Increasing poke brings the wheel closer to the fender edge for a "flush" look.

Practical Example

If you currently have an 8.0″ ET35 wheel and upgrade to a 9.0″ ET25 wheel:

  • The extra 1 inch of width is split: 12.7mm added to the inside, 12.7mm added to the outside.
  • The 10mm lower offset (35 to 25) pushes the whole wheel 10mm outward.
  • Result: You lose 2.7mm of inner clearance (12.7mm wider – 10mm push) and the wheel pokes out 22.7mm more (12.7mm wider + 10mm push).

Safety Warnings

When using this calculator, keep these safety margins in mind:

  1. Suspension Gap: Generally, you want at least 3-5mm of clearance between the wheel/tire and the strut.
  2. Fender Clearance: Ensure the wheel doesn't poke so far that it contacts the fender during suspension compression or while turning.
  3. Tire Sizes: Remember that the tire width also plays a massive role. A wider wheel often requires a wider tire, which might rub even if the wheel itself clears.
function calculateWheelFit() { var curW = parseFloat(document.getElementById('curWidth').value); var curO = parseFloat(document.getElementById('curOffset').value); var newW = parseFloat(document.getElementById('newWidth').value); var newO = parseFloat(document.getElementById('newOffset').value); if (isNaN(curW) || isNaN(curO) || isNaN(newW) || isNaN(newO)) { alert("Please enter valid numerical values for all fields."); return; } // Convert inches to mm (1 inch = 25.4mm) // Formula: Inner clearance change = (New Width/2 + New Offset) – (Old Width/2 + Old Offset) // Positive result = loses clearance (moves closer to strut) // Negative result = gains clearance (moves away from strut) var innerChange = (newW * 12.7 + newO) – (curW * 12.7 + curO); // Formula: Outer poke change = (New Width/2 – New Offset) – (Old Width/2 – Old Offset) // Positive result = pokes out further // Negative result = retracted further in var outerChange = (newW * 12.7 – newO) – (curW * 12.7 – curO); var innerResText = ""; var innerBox = document.getElementById('innerClearanceBox'); if (innerChange > 0) { innerResText = Math.abs(innerChange).toFixed(1) + "mm LESS clearance (closer to strut)"; innerBox.style.backgroundColor = "#fff3cd"; innerBox.style.color = "#856404"; } else if (innerChange 0) { outerResText = "EXTENDS " + Math.abs(outerChange).toFixed(1) + "mm further out"; outerBox.style.backgroundColor = "#d1ecf1"; outerBox.style.color = "#0c5460"; } else if (outerChange < 0) { outerResText = "RETRACTS " + Math.abs(outerChange).toFixed(1) + "mm further in"; outerBox.style.backgroundColor = "#f8d7da"; outerBox.style.color = "#721c24"; } else { outerResText = "No change in outer position"; outerBox.style.backgroundColor = "#e2e3e5"; outerBox.style.color = "#383d41"; } document.getElementById('innerResult').innerHTML = innerResText; document.getElementById('outerResult').innerHTML = outerResText; document.getElementById('resultsArea').style.display = 'block'; }

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