Before 1990, most cars had carburetors, which used vacuum to (badly) adjust the fuel/air ratio so the gas would ignite. One problem with carburetors is that when you accelerate hard , the F/A goes very lean, increasing combustion temperature and detonation. To 'solve' this most (all?) carbs had Power valves to make sure you were rich during acceleration to keep temps (and detonation) in check. Additionally accelerator pumps poured in pure gas to prevent a 'stumble' when first accelerating. Gentle acceleration did not engage these systems and was wildly more fuel efficient than brisk acceleration. No car sold in the US had a carb since 1995, these reasons for gentle acceleration have not existed in 20 years. With Fuel Injection we have new concepts: Open Loop and Closed loop When the car first starts, before the Catalytic converter warms up, the fuel injection is in an Open Loop, no feed back from the O2 sensors are telling the computers how to fine tune the F/A mixture. The computer uses a map, a table of predetermined and stored values to keep near perfect F/A ratios. Very strong acceleration may also make the ECU choose an open loop mode. Some ECU codes will throw the car in a Open Loop 'limp home' mode. Once the Prius is warmed up, the Fuel Injection starts using feedback from the O2 sensors to keep a perfect F/A ratio, always adding the correct amount of gas in a Closed feedback Loop. While your car is in a Closed Loop ideal economy emissions, and efficiency are in constant tune. Now if you 'peel rubber' you are still wasting gas, and if you accelerate so hard your computer goes Open Loop, you are wasting gas. (The Scangauge II can tell you when your Prius is open loop or closed loop, but it is dull as it is almost always closed loop) Other than that, getting to a set speed will cost the same amount of energy. (minus efficiency losses) Accelerating so hard you suck power out of the battery, then have to recharge the battery, would be an example of one such efficiency loss. Accelerating so soft you are trying to pull air passed an almost closed throttle plate, would be another example of efficiency loss.