Gear Ratio / RPM-Speed Calculator

Inspectable records

Ledger state

The current ledger marks this instrument verified clean within the source-bound verification corpus.

Area

Everyday & Life

Tranche

tranche-4

Evidence ID

t4-139

Ledger rows

1

No ledger note is attached.

Review findings

The forward formula is mathematically valid under the stated unit and ratio assumptions, but the specification needs explicit input units, the inverse RPM formula, ratio convention, validation constraints, and a clearer honest label. Verdict is NEEDS_REVIEW because these ambiguities materially affect results.

Formula and scope findings

• The forward speed formula is standard and correct when tire circumference is in feet, gear ratio and final drive are dimensionless reduction ratios, and RPM is rotational speed at the transmission input.
• The claimed reverse calculation is not explicitly provided. The required-RPM formula should be RPM = (speed_mph × gear_ratio × final_drive × 5280)/(tire_circumference_ft × 60).
• The circumference formula is correct as circumference = π × diameter only when diameter and circumference use the same length unit; the specification does not state whether tire diameter is supplied in inches, feet, millimeters, or another unit.
• Gear-ratio convention is ambiguous. The formula requires reduction ratio as input speed divided by output speed; using the inverse convention produces incorrect results.
• The formula assumes a fixed mechanical connection between engine/transmission RPM and tire speed. Automatic-transmission torque-converter slip, clutch slip, tire deformation, and tire growth are not represented.

Unit assumptions

• RPM is revolutions per minute.
• Vehicle speed is in statute miles per hour.
• Tire circumference must be in feet when using the factor 5280 feet per mile.
• Tire diameter must be converted to feet before calculating tire_circumference_ft; for example, diameter_ft = diameter_in / 12 or diameter_ft = diameter_mm / 304.8.
• Gear ratio and final drive ratio are dimensionless reduction ratios, with values greater than zero.
• π is treated as 3.141592653589793.

Boundary hardening

BREAK_CLAIMED · SELF_FLAGGED

A zero gear ratio creates an undefined division, and an unspecified tire-diameter unit can change the calculated speed by a factor of 12.

Errata and corrections

No applied erratum is attached to this unique-name record.

Inspectable example

4 review vectors are recorded. One source-bound sample is shown below.

{
  "inputs": {
    "mode": "speed_at_rpm",
    "rpm": 3000,
    "gear_ratio": 3.5,
    "final_drive": 3.73,
    "tire_diameter_in": 26
  },
  "expected_outputs": {
    "speed_mph": 17.7748
  },
  "working": "Convert diameter: 26 in ÷ 12 = 2.1666667 ft. Circumference = π × 2.1666667 = 6.8067841 ft. Speed = (3000 × 6.8067841 × 60) ÷ (3.5 × 3.73 × 5280) = 1,225,221.1349 ÷ 68,930.4 = 17.7748 mph."
}

Related workflows

No source-defined multi-instrument workflow membership is recorded.

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json · VERIFIED_CLEAN
• evidence-tranche4/tranche4-results.json
• BOUNDARY_HAZARD_CATALOG.json