Why are there two MPG values?

Because the US and UK gallons are not the same size. A UK gallon is about 20% larger, so the same car always shows a higher UK MPG figure. The converter keeps both so a review or a spec sheet written in either country can be read without guessing.

Which direction is better?

Higher km/L and higher MPG are better; lower L/100 km is better. That inversion is the usual reason a plain percentage change is confusing — a 10% drop in L/100 km is an improvement, not a loss.

How is L/100 km converted?

It is the reciprocal relationship: L/100 km equals 100 divided by km/L, and km/L equals 100 divided by L/100 km. That is why the two curves are not linear and small L/100 km figures turn into very large km/L figures.

What are the exact MPG factors?

This implementation multiplies km/L by `2.352145833` for US MPG and `2.824809` for UK MPG. Those factors are rounded constants in the code, not a claim of exact precision beyond the displayed digits.

Is the result rounded?

Each value is rounded to at most three decimal places for display, with trailing zeros omitted. The calculation happens before this display rounding.

Fuel Economy Converter

Value Unit

Unit	Value

Type a figure and pick the unit it is in; every other unit updates as you type. Lower L/100 km is better, while higher km/L and MPG are better, so the direction of a "good" number flips with the unit. US and UK gallons differ, which is why the two MPG figures are never the same.

Learn

Fuel Economy Converter Guide

About this tool

A one-in, four-out fuel economy converter. Type a figure, choose the unit it is in, and km/L, L/100 km and both MPG figures update as you type. All four units are defined by an exact conversion to km/L and back, so a value and its round trip agree. The maths runs in the page and nothing is uploaded.

Questions

Why are there two MPG values?: Because the US and UK gallons are not the same size. A UK gallon is about 20% larger, so the same car always shows a higher UK MPG figure. The converter keeps both so a review or a spec sheet written in either country can be read without guessing.
Which direction is better?: Higher km/L and higher MPG are better; lower L/100 km is better. That inversion is the usual reason a plain percentage change is confusing — a 10% drop in L/100 km is an improvement, not a loss.
How is L/100 km converted?: It is the reciprocal relationship: L/100 km equals 100 divided by km/L, and km/L equals 100 divided by L/100 km. That is why the two curves are not linear and small L/100 km figures turn into very large km/L figures.
What are the exact MPG factors?: This implementation multiplies km/L by 2.352145833 for US MPG and 2.824809 for UK MPG. Those factors are rounded constants in the code, not a claim of exact precision beyond the displayed digits.
Is the result rounded?: Each value is rounded to at most three decimal places for display, with trailing zeros omitted. The calculation happens before this display rounding.

Fuel Economy Converter