Let old price was P dollars per gallon.
Let old consumption was G gallons.


New price is 1.3*P.
New consumption is x gallons.


The equation is


1.3*P*x = P*G.     (1)


Hence,  x =  = 0.769*G.


Answer.  New consumption must be  0.769*G,  or   part of the old consumption.

     Equation (1) - your basic equation.

     And also the logic behind this solution, which leads to equation (1).