I conducted two tests on two different trips to our casino, which is 150 miles one way. the speed was set at 72 mph each time. gas tank filled each time. my vehicle is a 2000 Toyota Soara with a 4-cyliner engine, 4-speed automatic with a 5th gear overdrive. the first trip, i used the cruise control feature. the second trip, i manually drove the car. 60% of the interstate was flat. 40% was hilly. on the cruise control trip, i noticed my cars transmission was constantly changing gears as we going up the hilly sections. from 5th to 4th and sometimes 3rd gear. the cruise control was attempting to maintain the 72 mph speed, previously set. on the manual trip, i drove normally. after each trip, i filled tha gas tank and compared the two figures. the cruise control trip was 28 mpg and the manual driving was 31 mpg. i am really not complaining about either gas mileage. i am just wondering what made the difference?