# 燃費換算（mpg から L/100 km）

> 米ガロンと英ガロンに対応し、mpg、L/100 km、km/L を相互換算します。式：L/100 km = 235.215 ÷ US mpg または 282.481 ÷ UK mpg。

操作できるページ：https://www.calcopenly.com/ja/conversion/fuel-economy-converter
分野：単位換算ツール

Fuel economy comes in two shapes. Miles per gallon and kilometres per litre measure distance per unit of fuel; litres per 100 km measures fuel per distance. The two are reciprocals, so L/100 km = 100 ÷ (km/L), and a US mpg figure converts as L/100 km = 235.215 ÷ mpg.

The default converts 30 US mpg to 7.84 L/100 km. Comparing cars across markets is the usual reason: the US quotes mpg with a 3.785 L gallon, the UK quotes mpg with a 4.546 L gallon while selling fuel by the litre, most of Europe uses L/100 km and India uses km/L.

Because the scales are reciprocal, equal steps in mpg are not equal fuel savings: going from 10 to 20 mpg saves 5 gallons per 100 miles, while going from 30 to 50 mpg saves 1.33. The chart plots that curve.

## 入力

- **値**: 変換元の単位で数値を入力するか、単位付きの数値を入力します
- **変換元** (選択肢：kilometre per litre (km/L), litre per 100 km (L/100 km), mile per US gallon (mpg (US)), mile per imperial gallon (mpg (UK)))
- **変換先** (選択肢：kilometre per litre (km/L), litre per 100 km (L/100 km), mile per US gallon (mpg (US)), mile per imperial gallon (mpg (UK)))

## 結果

- 変換後の値 — 主な結果
- 単位換算
- 換算係数
- 正確さ

## 計算式

$$
\begin{gathered} \text{L/100 km} = \dfrac{100}{\text{km/L}} \\ \text{km/L} = \text{mpg}_{\text{US}} \times \dfrac{1.609344}{3.785411784} = \text{mpg}_{\text{UK}} \times \dfrac{1.609344}{4.54609} \\ \text{L/100 km} \approx \dfrac{235.215}{\text{mpg}_{\text{US}}} \approx \dfrac{282.481}{\text{mpg}_{\text{UK}}} \end{gathered}
$$

## 計算例

### 30 mpg (US) in L/100 km

- 値: 30 mpg (US)
- 変換元: mile per US gallon (mpg (US))
- 変換先: litre per 100 km (L/100 km)
- **変換後の値: 7.8404861111 L/100 km**
- **正確さ: 定義は正確、結果は循環小数**
- 照合元：100 ÷ (30 × 1.609344 km / 3.785411784 L) = 112903/14400 (Python 3.8 fractions from the legal definitions)

### 10 L/100 km in mpg (US)

- 値: 10 L/100 km
- 変換元: litre per 100 km (L/100 km)
- 変換先: mile per US gallon (mpg (US))
- **変換後の値: 23.5214583333 mpg (US)**
- 照合元：(100 / 10) km/L ÷ (1.609344/3.785411784) = 112903/4800 (Python 3.8 fractions from the legal definitions)

### 40 mpg (UK) in mpg (US)

- 値: 40 mpg (UK)
- 変換元: mile per imperial gallon (mpg (UK))
- 変換先: mile per US gallon (mpg (US))
- **変換後の値: 33.3069673852 mpg (US)**
- 照合元：40 × 3.785411784 / 4.54609 (Python 3.8 fractions from the legal definitions)

### Crossover: 10 km/L equals 10 L/100 km

- 値: 10 km/L
- 変換元: kilometre per litre (km/L)
- 変換先: litre per 100 km (L/100 km)
- **変換後の値: 10 L/100 km**
- **正確さ: 正確**
- 照合元：100 / 10 = 10 — the one value where km/L and L/100 km coincide

## よくある質問

### How do I convert mpg to L/100 km?

Divide 235.215 by a US mpg figure, or 282.481 by a UK mpg figure. 30 US mpg is 7.84 L/100 km and 50 UK mpg is 5.65 L/100 km. The constants are 100 × 3.785411784 ÷ 1.609344 and 100 × 4.54609 ÷ 1.609344, built from the exact gallon and mile.

### Why is UK mpg higher than US mpg for the same car?

Because the imperial gallon is 20% larger: 4.54609 L against 3.785411784 L. A car doing 40 mpg on UK figures does 33.3 mpg in US gallons, and 40 US mpg is 48.0 UK mpg. Both describe the same fuel use; 40 UK mpg is 7.06 L/100 km.

### Why does going from 10 to 20 mpg save more fuel than 30 to 50 mpg?

Because fuel used is distance divided by mpg. Over 100 miles, 10 mpg burns 10 gallons and 20 mpg burns 5, a saving of 5; 30 mpg burns 3.33 and 50 mpg burns 2, a saving of 1.33. Larrick and Soll called this the “MPG illusion” (Science, 2008); L/100 km avoids it because it measures fuel directly.

### How do I convert km/L to L/100 km?

Divide 100 by the km/L figure: 20 km/L is 5 L/100 km and 12.5 km/L is 8 L/100 km. At 10 km/L the two numbers are equal. To go from km/L to US mpg, multiply by 2.35215, so 15 km/L is 35.3 US mpg.

### Is a lower L/100 km better?

Yes. L/100 km counts fuel used, so a smaller number means less fuel: 5 L/100 km is twice as economical as 10 L/100 km. With mpg and km/L a larger number is better. The product of the two stays fixed, 235.215 for L/100 km and US mpg, which is why one falls as the other rises.

### 「燃費換算（mpg から L/100 km）」の精度はどのくらいですか？

精度は入力値と計算方法の前提に依存します。十進演算には有効数字50桁を使いますが、推定、数値計算手法、元データの精度はそれより低い場合があります。表示の丸め処理でこれらの制約がなくなるわけではありません。 独立した出典の解答と照合した計算例：4。 例えば、「30 mpg (US) in L/100 km」は100 ÷ (30 × 1.609344 km / 3.785411784 L) = 112903/14400 (Python 3.8 fractions from the legal definitions)と照合しています。

### この計算方法の出典は何ですか？

NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors; NIST Handbook 44, Appendix C — General tables of units of measurement.

## 出典

- [NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C — General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)