Tire Offset Calculator

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Tire Offset & Wheel Fitment Calculator

Current Wheel Specs

New Wheel Specs

Comparison Results

function calculateFitment() { var oldW = parseFloat(document.getElementById('oldWidth').value); var oldO = parseFloat(document.getElementById('oldOffset').value); var newW = parseFloat(document.getElementById('newWidth').value); var newO = parseFloat(document.getElementById('newOffset').value); if (isNaN(oldW) || isNaN(oldO) || isNaN(newW) || isNaN(newO)) { alert("Please enter valid numbers for all fields."); return; } // Calculations // Inner clearance change: (NewWidth/2 + NewOffset) – (OldWidth/2 + OldOffset) // Note: 1 inch = 25.4mm var innerChange = ((newW * 25.4) / 2 + newO) – ((oldW * 25.4) / 2 + oldO); // Outer extension change: (NewWidth/2 – NewOffset) – (OldWidth/2 – OldOffset) var outerChange = ((newW * 25.4) / 2 – newO) – ((oldW * 25.4) / 2 – oldO); var innerResult = document.getElementById('innerClearance'); var outerResult = document.getElementById('outerPoke'); var summary = document.getElementById('summaryText'); // Display Logic for Inner Clearance if (innerChange > 0) { innerResult.innerHTML = "Inner Clearance: " + Math.abs(innerChange).toFixed(1) + "mm LESS clearance (wheel is closer to suspension)."; } else if (innerChange < 0) { innerResult.innerHTML = "Inner Clearance: " + Math.abs(innerChange).toFixed(1) + "mm MORE clearance (wheel is further from suspension)."; } else { innerResult.innerHTML = "Inner Clearance: No change."; } // Display Logic for Outer Extension (Poke) if (outerChange > 0) { outerResult.innerHTML = "Outer Extension: Wheel will poke out " + Math.abs(outerChange).toFixed(1) + "mm FURTHER than current."; } else if (outerChange < 0) { outerResult.innerHTML = "Outer Extension: Wheel will sit " + Math.abs(outerChange).toFixed(1) + "mm TUCKED IN more than current."; } else { outerResult.innerHTML = "Outer Extension: No change."; } summary.innerHTML = "Comparison based on a " + oldW + "\" ET" + oldO + " wheel vs. a " + newW + "\" ET" + newO + " wheel."; document.getElementById('resultsArea').style.display = 'block'; }

Understanding Tire Offset and Wheel Fitment

Changing your wheels is one of the most impactful modifications you can make to your vehicle. However, it is more than just picking a pretty design. You must understand wheel offset (often marked as ET) to ensure your new tires don't rub against the suspension components or stick out too far from the wheel arches.

What is Wheel Offset?

Wheel offset is the distance from the hub-mounting surface to the centerline of the wheel. It is measured in millimeters and results in three categories:

  • Positive Offset: The mounting surface is toward the front (street side) of the wheel. Most modern front-wheel-drive vehicles use positive offset.
  • Zero Offset: The mounting surface is even with the centerline of the wheel.
  • Negative Offset: The mounting surface is toward the back (braking side) of the wheel's centerline. This "deep dish" look is common on lifted trucks and older rear-wheel-drive cars.

How the Offset Calculator Works

When you increase wheel width or change the offset, the wheel moves in two directions: inward toward the strut and outward toward the fender. Our calculator uses the following logic:

  1. Inner Clearance: We calculate the distance from the hub to the inner edge. If this number increases, your wheel might hit the strut or coilover.
  2. Outer Poke: We calculate the distance from the hub to the outer edge. If this number increases significantly, you may need to roll your fenders or add flares to prevent rubbing on the tire sidewall.

Example Fitment Scenario

Imagine you are upgrading from a stock 18×8.0″ ET45 wheel to a more aggressive 18×9.0″ ET35 setup:

  • Width Change: The new wheel is 1 inch (25.4mm) wider.
  • Offset Change: The offset decreased by 10mm (moving the wheel outward).
  • Result: Your wheel will have 2.7mm less room on the inside (near the suspension) but will poke out 22.7mm further toward the fender.

Common Fitment Terms

Backspacing: This is the distance from the mounting surface to the back edge of the wheel. While offset is the standard for cars, backspacing is frequently used in the off-road and classic car communities.

Scrub Radius: Changing your offset alters the scrub radius, which can affect steering feel, feedback, and how the car tracks over bumps. Drastic changes in offset can lead to increased wear on wheel bearings.

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