# Fuel Economy Converter

> Converts fuel consumption between litres per 100 km, kilometres per litre, miles per US gallon and miles per imperial gallon; L/100km is the base and the mpg relations are reciprocal (L/100km = 235.214583 / mpg_US).

- Calculator id: `fuel-economy` · Category: Unit Conversion (`conversion`) · Tool name: `convert_fuel_economy`
- Canonical page: https://tttkmbb.com/conversion/fuel-economy · This document: https://tttkmbb.com/conversion/fuel-economy.md · JSON definition: https://tttkmbb.com/conversion/fuel-economy.json

## Purpose

Converts fuel consumption between litres per 100 km, kilometres per litre, miles per US gallon and miles per imperial gallon; L/100km is the base and the mpg relations are reciprocal (L/100km = 235.214583 / mpg_US).

**Use when:** You need a car's fuel economy in another convention, e.g. mpg to L/100km, L/100km to km/L or US mpg to UK mpg.

**Do not use when:** You need the fuel cost of a trip (use fuel-cost) or plain volume or distance conversions (use volume, length).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `value` | number |  | required | Fuel economy in from_unit (must be greater than 0). (> 0) |
| `from_unit` | enum: L/100km \| km/L \| mpg_us \| mpg_uk |  | required | Unit of the input value. Accepted symbols: L/100km, km/L, mpg_us, mpg_uk; spelled-out names and common abbreviations are accepted too. |
| `to_unit` | enum: L/100km \| km/L \| mpg_us \| mpg_uk |  | required | Unit to convert into (same symbols as from_unit). |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `result` | number |  | Converted value expressed in to_unit (rounded to 6 decimals; see result_text for very small or very large values). |
| `result_text` | string |  | The conversion as text with 6 significant figures, e.g. "5 mi = 8.04672 km". |
| `formula` | string |  | Relation used, e.g. "1 mi = 1.609344 km". |
| `factor` | number |  | Multiplier from from_unit to to_unit (result = value × factor); absent for non-linear conversions. |
| `conversion_table` | object |  | The input value expressed in every supported fuel economy unit (6 significant figures), keyed by unit symbol. |

## Formula

`L/100km = 100 / (km/L) = 235.214583 / mpg(US) = 282.480936 / mpg(UK); mpg(UK) = mpg(US) × 1.200950`

Consumption (L/100km) and economy (km/L, mpg) are reciprocals, so the conversion is non-linear and no constant factor applies. Constants: 100 × 3.785411784 / 1.609344 = 235.214583 and 100 × 4.54609 / 1.609344 = 282.480936.

## Data Sources

- NIST Special Publication 811 – Guide for the Use of the International System of Units (SI), Appendix B conversion factors — https://www.nist.gov/pml/special-publication-811 (standard, retrieved 2026-09-23)
- Wikipedia – Fuel economy in automobiles (units and conversions) — https://en.wikipedia.org/wiki/Fuel_economy_in_automobiles (reference, retrieved 2026-09-23)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/fuel-economy?value=…&from_unit=…&to_unit=…`
- `POST https://tttkmbb.com/api/v1/calculate/fuel-economy` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/fuel-economy · OpenAPI operationId `convert_fuel_economy` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "fuel-economy", "inputs": {…}}`

## Example

- 30 US mpg to L/100km: inputs `{"value":30,"from_unit":"mpg_us","to_unit":"L/100km"}` → `{"result":7.840486,"formula":"L/100km = 235.214583 / mpg (US)","conversion_table":{"km/L":12.7543,"mpg_uk":36.0285}}`
- 6 L/100km to US mpg: inputs `{"value":6,"from_unit":"L/100km","to_unit":"mpg_us"}` → `{"result":39.202431,"conversion_table":{"km/L":16.6667,"mpg_uk":47.0802}}`

```
GET https://tttkmbb.com/api/v1/calculate/fuel-economy?value=30&from_unit=mpg_us&to_unit=L%2F100km
```

## Limitations

You need the fuel cost of a trip (use fuel-cost) or plain volume or distance conversions (use volume, length). Consumption (L/100km) and economy (km/L, mpg) are reciprocals, so the conversion is non-linear and no constant factor applies. Constants: 100 × 3.785411784 / 1.609344 = 235.214583 and 100 × 4.54609 / 1.609344 = 282.480936. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Which mpg does 'mpg' mean?**

The plain alias mpg resolves to miles per US gallon (mpg_us). UK figures use the larger imperial gallon: mpg_uk = mpg_us × 1.20095.

**Why is lower L/100km better?**

L/100km measures consumption, mpg and km/L measure economy; 5 L/100km = 20 km/L = 47.0 US mpg, and halving consumption doubles mpg.

## Related

- [Fuel Cost Calculator](https://tttkmbb.com/everyday/fuel-cost.md) — Trip fuel cost from distance, consumption and price.
- [Volume Converter](https://tttkmbb.com/conversion/volume.md) — Convert gallons and litres directly.
- [Length Converter](https://tttkmbb.com/conversion/length.md) — Convert miles and kilometres directly.