Passenger miles per gallon (PMPG) is a metric

Metric (mathematics)

In mathematics, a metric or distance function is a function which defines a distance between elements of a set. A set with a metric is called a metric space. A metric induces a topology on a set but not all topologies can be generated by a metric...

 to evaluate the energy efficiency

Energy conversion efficiency

Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...

 of a vehicle or transportation mode. The PMPG can be obtained by multiply

Multiply

Multiply is a social networking service with an emphasis on allowing users to share media – such as photos, videos and blog entries – with their "real-world" network...

ing the miles per gallon of fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 (MPG) by the passenger capacity

Seating capacity

Seating capacity refers to the number of people who can be seated in a specific space, both in terms of the physical space available, and in terms of limitations set by law. Seating capacity can be used in the description of anything ranging from an automobile that seats two to a stadium that seats...

or average occupancy. The occupancy of personal vehicles is typically lower than capacity by a considerable degree and thus the values computed based on capacity and on occupancy will often be quite different.