Fuel consumption depends more on the driver than the vehicle. A person driving very economically from the start may not see a significant difference, while conversely, a driver who uses the accelerator pedal more aggressively will notice a quicker decrease.
Indeed, to achieve the same accelerations before and after reprogramming, applying less pressure on the accelerator will result in similar accelerations and RPMs as the original state.
Accelerating more gently consumes less fuel since consumption is directly related to the demanded load.
It also depends on the engine type, engine management, or the selected tuning stage.
A reduction in fuel consumption is more likely to occur in a diesel vehicle than in a gasoline vehicle.
Therefore, it is impossible to guarantee a fuel consumption improvement, but it is not uncommon to observe a decrease.