{
  "slug": "tire-size-comparison",
  "title": "Tire Size Calculator",
  "heading": "Tire Size Comparison Calculator",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/tire-size-comparison",
  "summary": "Diameter, circumference, and speedometer error between two tire sizes.",
  "description": "Changing tire size changes your effective final drive. More than about 3% difference in rolling diameter and your speedometer and odometer are meaningfully off.",
  "formula": "Diameter (in) = Rim + 2 × (Width mm × Aspect% ÷ 25.4 ÷ 100).",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=tire-size-comparison",
  "inputs": [
    {
      "id": "w1",
      "label": "Original width (mm)",
      "kind": "number",
      "hint": null,
      "default": 225,
      "unit": null,
      "min": 100,
      "max": null
    },
    {
      "id": "a1",
      "label": "Original aspect ratio",
      "kind": "number",
      "hint": null,
      "default": 45,
      "unit": null,
      "min": 20,
      "max": 90
    },
    {
      "id": "r1",
      "label": "Original rim (in)",
      "kind": "number",
      "hint": null,
      "default": 17,
      "unit": null,
      "min": 10,
      "max": 24
    },
    {
      "id": "w2",
      "label": "New width (mm)",
      "kind": "number",
      "hint": null,
      "default": 245,
      "unit": null,
      "min": 100,
      "max": null
    },
    {
      "id": "a2",
      "label": "New aspect ratio",
      "kind": "number",
      "hint": null,
      "default": 40,
      "unit": null,
      "min": 20,
      "max": 90
    },
    {
      "id": "r2",
      "label": "New rim (in)",
      "kind": "number",
      "hint": null,
      "default": 18,
      "unit": null,
      "min": 10,
      "max": 24
    },
    {
      "id": "speed",
      "label": "Indicated speed (mph)",
      "kind": "number",
      "hint": null,
      "default": 60,
      "unit": null,
      "min": 0,
      "max": null
    }
  ],
  "outputs": [
    {
      "id": "diff",
      "label": "Diameter difference",
      "format": "percent",
      "hint": null,
      "primary": true
    },
    {
      "id": "d1",
      "label": "Original diameter (in)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "d2",
      "label": "New diameter (in)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "actualSpeed",
      "label": "True speed at indicated speed (mph)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "c2",
      "label": "New tire circumference (in)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "revsPerMile",
      "label": "Revolutions per mile",
      "format": "number",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Original width (mm): 225",
      "Original aspect ratio: 45",
      "Original rim (in): 17",
      "New width (mm): 245",
      "New aspect ratio: 40",
      "New rim (in): 18",
      "Indicated speed (mph): 60"
    ],
    "outputs": [
      "Diameter difference: 2.98%",
      "Original diameter (in): 24.97",
      "New diameter (in): 25.72",
      "True speed at indicated speed (mph): 61.8",
      "New tire circumference (in): 80.79",
      "Revolutions per mile: 784.25"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter original width (mm).",
      "Enter original aspect ratio.",
      "Enter original rim (in).",
      "Enter new width (mm).",
      "Enter new aspect ratio.",
      "Enter new rim (in).",
      "Enter indicated speed (mph).",
      "Read your diameter difference on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "w1": 135,
        "a1": 25,
        "r1": 10,
        "w2": 145,
        "a2": 25,
        "r2": 11,
        "speed": 35
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "w1": 225,
        "a1": 45,
        "r1": 17,
        "w2": 245,
        "a2": 40,
        "r2": 18,
        "speed": 60
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "w1": 360,
        "a1": 70,
        "r1": 24,
        "w2": 390,
        "a2": 65,
        "r2": 24,
        "speed": 95
      }
    }
  ],
  "limitations": [
    "Estimates assume standard, average conditions — local rules, pricing, and materials vary.",
    "Results are rounded for readability; add a buffer before ordering, booking, or committing.",
    "Double-check anything with a real cost attached against a local quote."
  ],
  "faq": [
    {
      "q": "How does the tire size comparison calculator work?",
      "a": "Changing tire size changes your effective final drive. More than about 3% difference in rolling diameter and your speedometer and odometer are meaningfully off. The underlying maths is: Diameter (in) = Rim + 2 × (Width mm × Aspect% ÷ 25.4 ÷ 100)."
    },
    {
      "q": "What do I need to enter?",
      "a": "7 values: original width (mm), original aspect ratio, original rim (in), new width (mm), new aspect ratio, new rim (in), and indicated speed (mph). Each field starts with a sensible default, so you can change one number at a time and watch the result move."
    },
    {
      "q": "What does the diameter difference result mean?",
      "a": "It is returned as a percentage and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds."
    },
    {
      "q": "Is this calculator free, and do I need an account?",
      "a": "Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser."
    },
    {
      "q": "How accurate is the result?",
      "a": "It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary."
    },
    {
      "q": "Who is this tool for?",
      "a": "It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/mpg",
    "https://www.revenuelab.fyi/toolbox/cost-per-mile"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Tire Size Calculator (https://www.revenuelab.fyi/toolbox/tire-size-comparison)"
}