Source: https://guthlabs.ai/docs/instruments/tire-size-comparator/mechanism/

instrument mechanism / verified-clean

# Tire Size Comparator mechanism

The inspectable method record for Tire Size Comparator, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The nominal diameter formula is standard and the unit conversion is correct. The instrument should explicitly define ground-clearance change as half the diameter delta and name the speedometer-error convention before release. The existing educational-estimate label is appropriate but should retain the fitment and load-rating warning.

- Ground-clearance change is not explicitly defined. The standard nominal assumption is ground_clearance_change = (new_diameter - old_diameter) / 2, but this formula is absent from the specification.

- speedo_error% = (new_diameter - old_diameter) / old_diameter × 100 is only one possible convention. It represents the actual-speed change at the old indicated speed, not necessarily speedometer error relative to actual speed. The reference convention must be named.

- The formulas calculate nominal unloaded geometric diameter; actual tire radius and ground clearance vary with tire deflection, load, pressure, and construction.

## Units and conventions

- Section width is in millimeters, aspect ratio is a percentage, and rim diameter is in inches.

- 25.4 millimeters per inch is used to convert rim diameter to millimeters.

- Ground-clearance change is assumed to equal half the nominal diameter change, with vehicle and wheel-center geometry otherwise unchanged.

- Tire circumference is assumed proportional to nominal diameter, so diameter ratio is used for the speed comparison.

- The percentage difference is signed from old tire to new tire.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "old_tire": "225/45R17",
    "new_tire": "235/40R18"
  },
  "expected_outputs": {
    "old_diameter_mm": 634.3,
    "new_diameter_mm": 645.2,
    "diameter_delta_mm": 10.9,
    "percent_difference": 1.71843,
    "speedometer_error_percent_same_ratio": 1.71843,
    "ground_clearance_change_mm": 5.45
  },
  "working": "Old sidewall = 225 × (45/100) = 101.25 mm. Old rim = 17 × 25.4 = 431.8 mm. Old diameter = 431.8 + 2 × 101.25 = 634.3 mm. New sidewall = 235 × (40/100) = 94 mm. New rim = 18 × 25.4 = 457.2 mm. New diameter = 457.2 + 2 × 94 = 645.2 mm. Diameter delta = 645.2 − 634.3 = 10.9 mm. Percent difference = 10.9 ÷ 634.3 × 100 = 1.71843%. Ground-clearance change = 10.9 ÷ 2 = 5.45 mm. Speedometer same-ratio value = 1.71843%."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The formulas produce an undefined percentage when the old tire's computed diameter is zero, and they also accept physically invalid zero aspect ratios unless input validation rejects them. The speedometer-error label is convention-sensitive because the specification's ratio differs from the usual indicated-versus-true-speed interpretation.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/tire-size-comparator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/tire-size-comparator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/tire-size-comparator/qualification/>)

## Sources and evidence

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

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`