Ullage

To clarify, ullage is a noun which represents the amount of liquid missing from a container's full capacity. If your Suburban had a capacity of 40 gallons of gasoline, but only had 5 gallons in the tank, the ullage would be 35 gallons.

Ul"lage (?; 48), n. [OF. eullage, ovillage, the filling up of a cask, fr. ouillier, oillier, euillier, to fill a wine cask; properly, to add oil to prevent evaporation, as to a flask that is nearly full, fr. OF. oile oil. See Oil.] Com.

The amount which a vessel, as a cask, of liquor lacks of being full; wantage; deficiency.

© Webster 1913.