To convert miles per gallon (mpg) to kilometres per litre (km/l), you simply multiply the mpg value by $0.4251$.

To fully grasp this conversion, it’s essential to understand the relationships between miles and kilometres, as well as gallons and litres. Specifically, there are $1.60934$ kilometres in a mile and $3.78541$ litres in a gallon. These conversions are the foundation for converting mpg to km/l.

Let’s break down the conversion process step by step:

1. Miles to Kilometres: Since $1$ mile is approximately $1.60934$ kilometres, if a vehicle travels $1$ mile per gallon, it effectively travels $1.60934$ kilometres per gallon.

2. Gallons to Litres: Next, we convert gallons to litres. Given that $1$ gallon is approximately $3.78541$ litres, we can find the number of kilometres per litre by dividing the distance travelled in kilometres by the volume in litres. Thus, if a car travels $1.60934$ kilometres per gallon, the conversion to kilometres per litre is given by:

   $$\text{km/l} = \frac{1.60934 \text{ km}}{3.78541 \text{ l}} \approx 0.4251 \text{ km/l per mpg}$$

Therefore, to convert any fuel efficiency value from miles per gallon to kilometres per litre, you simply multiply the mpg figure by $0.4251$.

For instance, if a vehicle has a fuel efficiency of $30$ mpg, the calculation for its efficiency in kilometres per litre would be:

This straightforward method guarantees an accurate conversion of fuel efficiency from the imperial system to the metric system.