NOUN (3)

1. the quantity contained in (or the capacity of) a tank or tanks;

2. the charge for storing something in tanks;

3. the act of storing in tanks;

The Collaborative International Dictionary of English v.0.48:

Tankage \Tank"age\, n.
   1. The act or process of putting or storing in tanks.
      [Webster 1913 Suppl.]

   2. Fees charged for storage in tanks.
      [Webster 1913 Suppl.]

   3. The capacity or contents of a tank or tanks.
      [Webster 1913 Suppl.]

   4. (Agric.) Waste matter from tanks; esp., the dried
      nitrogenous residue from tanks in which fat has been
      rendered, used as a fertilizer.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tankage
    n 1: the quantity contained in (or the capacity of) a tank or
         tanks
    2: the charge for storing something in tanks
    3: the act of storing in tanks