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

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "mileage":
   account, aesthetic distance, allowance, assessment, bill,
   blackmail, blood money, clearance, compass, deep space,
   depths of space, distance, divergence, emolument, extension,
   extent, farness, fee, footing, hush money, infinity,
   initiation fee, leeway, length, lengthiness, light-years,
   linear measures, long time, longitude, longness, margin, measure,
   overall length, parsecs, perpetuity, perspective, piece, range,
   reach, reckoning, remoteness, retainer, retaining fee, scot,
   separation, space, span, stipend, stretch, stride, tribute, way,
   ways, yardage



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MILEAGE. A compensation allowed by law to officers, for their trouble and
expenses in travelling on public business.
     2. The mileage allowed to members of congress, is eight dollars for
every twenty miles of estimated distance, by the most usual roads, from his
place of residence to the seat of congress, at the commencement and end of
every session. Act of Jan. 22, 1818; 3 Story, Laws U. S. 1657.
     3. In computing mileage the distance by the road usually travelled is
that which must be allowed, whether in fact the officer travels a more or
less distant way to suit his own convenience. 5 Shepl. R. 431.