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diff --git a/reference/howto.mdown b/reference/howto.mdown
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+### 1. File containing the relevant code
+The logic is embedded directly within the HTML file you provided: `Tire Size Calculator.html` (specifically between lines 2944 and 2981).
+
+### 2. How the calculations are done
+The math is based on finding the circumference of the wheel (`Circumference = Pi * Diameter`). 
+1. **Total Diameter:** It takes the **Rim Diameter** (`r`) and adds it to twice the **Tire Width/Diameter** (`t`) because the tire sits on both the top and bottom of the rim. This gives the total wheel diameter: `(2 * t + r)`.
+2. **Circumference:** It multiplies the total diameter by `3.13772`. This is a slightly modified value of Pi (π ≈ 3.14159), which is commonly used in cycling to account for the tire compressing under the rider's weight (often called "roll-out" circumference).
+3. **Conversions:** Once the circumference is calculated in millimeters (`d`), it converts it to:
+   - **Centimeters:** Divides by 10.
+   - **Inches:** Divides by 25.4.
+   - **MPH Setting:** Divides by 1.6 (likely a specific calibration setting for their computers, converting km to miles).
+
+### 3. Relevant Code
+Here is the exact JavaScript snippet from the file that handles this logic:
+
+````javascript
+function Recalc() {
+    // Get Rim and Tire values in millimeters
+    var r = parseFloat(document.Calc.RimDiameter.value);
+    var t = parseFloat(document.Calc.TireDiameter.value);
+    
+    if (r > 0 && t > 0) {
+        // Calculate circumference in mm
+        var d = 3.13772 * (2 * t + r);
+        
+        // Output the rounded values
+        document.Calc.mm.value = Math.round(d);
+        document.Calc.cm.value = Math.round(d / 10);
+        document.Calc.inches.value = Math.round(10 * d / 25.4) / 10;
+        document.Calc.MPH.value = Math.round(d / 1.6);
+    } else {
+        // Clear fields if inputs are invalid
+        document.Calc.mm.value = '';
+        document.Calc.cm.value = '';
+        document.Calc.inches.value = '';
+        document.Calc.MPH.value = '';
+    }
+}
+````
diff --git a/reference/tire-sizes.json b/reference/tire-sizes.json
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+{
+  "wheel_sizes": [
+    { "label": "12.5\"", "diameter_mm": 203 },
+    { "label": "14\"", "diameter_mm": 254 },
+    { "label": "16\"", "diameter_mm": 305 },
+    { "label": "20\"", "diameter_mm": 406 },
+    { "label": "24\" BMX/Kids Bikes", "diameter_mm": 507 },
+    { "label": "24\" Terry Bicycles", "diameter_mm": 520 },
+    { "label": "24\"", "diameter_mm": 540 },
+    { "label": "24\" Schwinn", "diameter_mm": 540 },
+    { "label": "26\"", "diameter_mm": 559 },
+    { "label": "650c", "diameter_mm": 571 },
+    { "label": "650b", "diameter_mm": 584 },
+    { "label": "27.5", "diameter_mm": 584 },
+    { "label": "26 x 1 1/2\"", "diameter_mm": 584 },
+    { "label": "650a", "diameter_mm": 590 },
+    { "label": "26 x 1 3/8\"", "diameter_mm": 590 },
+    { "label": "650", "diameter_mm": 597 },
+    { "label": "26 x 1 1/4\"", "diameter_mm": 597 },
+    { "label": "29er", "diameter_mm": 622 },
+    { "label": "700c", "diameter_mm": 622 },
+    { "label": "27 inch", "diameter_mm": 630 }
+  ],
+  "tire_sizes": [
+    { "label": "18C", "diameter_mm": 18 },
+    { "label": "19C", "diameter_mm": 19 },
+    { "label": "20C", "diameter_mm": 20 },
+    { "label": "21C", "diameter_mm": 21 },
+    { "label": "23C", "diameter_mm": 23 },
+    { "label": "25C", "diameter_mm": 25 },
+    { "label": "26C", "diameter_mm": 26 },
+    { "label": "28C", "diameter_mm": 28 },
+    { "label": "30C", "diameter_mm": 30 },
+    { "label": "32C", "diameter_mm": 32 },
+    { "label": "35C", "diameter_mm": 35 },
+    { "label": "38C", "diameter_mm": 38 },
+    { "label": "40C", "diameter_mm": 40 },
+    { "label": "44C", "diameter_mm": 44 },
+    { "label": "50C", "diameter_mm": 50 },
+    { "label": "56C", "diameter_mm": 56 },
+    { "label": "1.00 inch", "diameter_mm": 25.4 },
+    { "label": "1 1/8 inch", "diameter_mm": 28.575 },
+    { "label": "1.25 inch", "diameter_mm": 31.75 },
+    { "label": "1 3/8 inch", "diameter_mm": 34.925 },
+    { "label": "1.4 inch", "diameter_mm": 35.56 },
+    { "label": "1.5 inch", "diameter_mm": 38.1 },
+    { "label": "1.75 inch", "diameter_mm": 44.45 },
+    { "label": "1.95 inch", "diameter_mm": 49.5 },
+    { "label": "2.00 inch", "diameter_mm": 50.8 },
+    { "label": "2.10 inch", "diameter_mm": 53.34 },
+    { "label": "2.125 inch", "diameter_mm": 54 },
+    { "label": "2.20 inch", "diameter_mm": 55.88 },
+    { "label": "2.25 inch", "diameter_mm": 57.15 },
+    { "label": "2.30 inch", "diameter_mm": 58.42 },
+    { "label": "2.35 inch", "diameter_mm": 59.69 },
+    { "label": "2.40 inch", "diameter_mm": 60.69 },
+    { "label": "2.50 inch", "diameter_mm": 63.5 },
+    { "label": "2.60 inch", "diameter_mm": 66.04 },
+    { "label": "2.70 inch", "diameter_mm": 68.58 },
+    { "label": "2.80 inch", "diameter_mm": 71.12 },
+    { "label": "2.90 inch", "diameter_mm": 73.66 },
+    { "label": "3.0 inch", "diameter_mm": 76.2 },
+    { "label": "3.1 inch", "diameter_mm": 78.74 },
+    { "label": "3.2 inch", "diameter_mm": 81.28 },
+    { "label": "3.3 inch", "diameter_mm": 83.82 },
+    { "label": "3.4 inch", "diameter_mm": 86.36 },
+    { "label": "3.5 inch", "diameter_mm": 88.9 },
+    { "label": "3.6 inch", "diameter_mm": 91.44 },
+    { "label": "3.7 inch", "diameter_mm": 93.98 },
+    { "label": "3.8 inch", "diameter_mm": 96.52 },
+    { "label": "3.9 inch", "diameter_mm": 99.06 },
+    { "label": "4.0 inch", "diameter_mm": 101.6 }
+  ]
+}