The Collaborative International Dictionary of English v.0.48:

Mileage \Mile"age\ (m[imac]l"[asl]j; 48), n.
   1. An allowance for traveling expenses at a certain rate per
      mile.
      [1913 Webster]

   2. Aggregate length or distance in miles; esp., the sum of
      lengths of tracks or wires of a railroad company,
      telegraph company, etc. [Written also milage.]
      [1913 Webster]

   3. The number of miles that a vehicle can travel after
      consuming a certain quantity of fuel; in the United
      States, usually expressed in units of miles per gallon;
      as, smaller cars tend to get better mileage. It is
      sometimes used as a nmeasure of the energy efficiency of a
      vehicle.
      [PJC]

   4. Use, profit or advantage; as, he got a lot of mileage out
      of one hit record.
      [PJC]

   Constructive mileage, a mileage allowed for journeys
      supposed to be made, but not actually made. --Bartlett.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mileage
    n 1: distance measured in miles [syn: mileage, milage]
    2: the ratio of the number of miles traveled to the number of
       gallons of gasoline burned [syn: mileage, fuel consumption
       rate, gasoline mileage, gas mileage]
    3: a travel allowance at a given rate per mile traveled