An analysis was performed on gas mileage at a local automotive testing ground. The purpose of the study was to try and determine whether tire pressure, timing setting, type of oil, and/or type of gas had significant effects on gas mileage. Timing settings were low and high. Types of oil were 10W30 and 10W40. Two types of gasoline were used: mid-grade and hi-test. Tire pressures were tested at levels of 30 and 34 psi. What is the response variable?