The biggest reason for the gap is that American and European gallons are different amounts. American gallons are smaller, resulting in a huge discrepancy in fuel economy.
The imperial gallon
The imperial (UK) gallon, defined as 4.54609 litres, is used in some Commonwealth countries and was originally based on the volume of 10 pounds of water at 62 °F (17 °C). (A US liquid gallon of water weighs about 8.34 pounds at the same temperature.) The imperial fluid ounce is defined as 1⁄160 of an imperial gallon.
The US liquid gallon
The US gallon, which is equal to 3.785411784 litres is legally defined as 231 cubic inches.[1][2] There are four quarts in a gallon, two pints in a quart and sixteen fluid ounces in a pint. In order to overcome the effects of expansion and contraction with temperature when using a gallon to specify a quantity of material for purposes of trade, it is common to define the temperature at which the material will occupy the specified volume. For example, for purposes of trade, the volume of petroleum products[3] and alcoholic beverages[4] are both referenced to 60 °F (16 °C).
The US dry gallon
This gallon is one-eighth of a US Winchester bushel of 2150.42 cubic inches, thus it is equal to exactly 268.8025 cubic inches or 4.40488377086 L. The US dry gallon is less commonly used, and is not listed in the relevant statute, which jumps from the dry quart to the peck.[5]
Example: Running my Saturn's highway rated gas mileage through a converter (35 MPG US) brings it to 42 MPG imperial. The 25 us MPG city becomes 30 MPG imperial.
My Z34:
US city: 18 MPG
US Highway: 26 MPG
imperial city: 21.6 MPG
imperial highway: 31.2 MPG
2013 Chevrolet Cruze Eco (1.4L turbo, 6 M/T)
US city: 28 MPG
US Highway: 42 MPG
imperial city: 33.6 MPG
imperial highway: 50.4 MPG
My Dad's 2011 Silverado (5.3L V8, 6 A/T)
US city: 15 MPG
US Highway: 22 MPG
imperial city: 18 MPG
imperial highway: 26.4 MPG
