In my 2018 Silverado V6 RWD, in case V4 fails to activate on flat road automatically, I just increase one speed on cruise control, and then decrease one speed back. Therefore, the V4 immediately activates. That's the trick.Unless you know some magic trick, the computer decides when to go into V4 mode, not the driver. I'm driven many times up and down I-81 at 70-80 mph for 300 miles. V4 mode very seldom activates. Even in V6 mode I still average 32-33 mpg. I'm not convinced that 45 mpg is attainable.