The term 'PART OF MPG' refers to the components of Miles PER Gallon, which is a measure of fuel efficiency for vehicles. The three crossword solutions for this term are PER (3 letters), GALLON (6 letters), and GAL (3 letters). PER stands for 'PER 100', which is the number of miles a vehicle can travel on 100 units of fuel. GALLON is a unit of measurement for liquid volume, and GAL is an abbreviation for gallon.