Switching to Sport or Power mode improves a car's acceleration. This is because the engine holds on to the gears up to a higher rpm. Invariably, fuel economy suffers because the engine revs harder. However, I find that when I drive gently in this mode - meaning feathering the accelerator pedal - the performance is brisk while fuel consumption improves. Why is this so?
The default setting for most cars is geared towards efficiency.
The transmission changes up to top gear as early as the speed permits in order to maintain the lowest possible engine speed.
Car manufacturers do this to ensure that the laboratory fuel consumption test figures are optimised.
In the real world and under relaxed driving conditions, the fuel consumption rarely ever approaches the laboratory figures.
Nonetheless, the car in Normal mode will return to the best possible km/litre or litres/100km numbers. However, you will find that acceleration is leisurely and you need to depress the accelerator pedal further if you want the gears to shift down for better acceleration.
By selecting Sport, you will need less throttle for acceleration because the car is already in a lower gear than it would have been at the same speed in Normal mode.
This style of driving can result in better fuel consumption figures. But it does require some practice to perfect.
Remember also that with most cars, the top gear in Sport mode is attained at a higher cruising speed. So for best fuel economy, it is advisable to revert to Normal once you get on the highway or are at a cruising pace.