Normal rule is to tuck your left foot away from pedals, i.e. use right foot only. There are a few exceptions: Class 1 Police drivers in pursuit conditions; Top Gear type performance driving; disabled driver. However, the DSA, who run the Driving Test, do recommend keeping the right foot over the brake pedal when manouevering in tight spots in case of sudden engine surge, particularly when the choke is operational. I reguarly read newspaper articles about (usually elderly) automatic drivers causing mayhem by pressing the gas pedal harder to stop in confusion despite the hugeness of an autos footbrake pedal. In normal driving in "D", gentle acceleration will give the best economy, hard acceleration in "sports mode" the worst. The auto gearboxes "brain" & the engine ECU should not allow over reving.