[IamIan]

Speed variation is always an aerodynamic penalty over maintaining the same average speed .. under any and all methods .. any speed .. etc... This is a consequence of the exponentially increasing wind resistance.

P&G works as a net vehicle benefit when some other effect of the P&G overcomes the aerodynamic penalty for speed fluctuations.

One of the big effects that can beat the wind resistance penalty many times in P&G is the net ICE operating efficiency... So look for the BSFC for your ICE and try and line up your P&G with the best net ICE operating efficiency.

As others have said , this is easier to achieve at slower speeds .. because the larger the aerodynamic penalty the harder it is to find some other drive method benefit to overcome them... and wind resistances increase very fast with faster speeds... Wind resistance force increases @ V^2 .. wind resistance power increases @ V^3 ... 10mph to 40 mph is 16x more wind resistance force and 64x more power... but only 4x faster.