Something like that may have been true in the days of the Power-Glide 2 speed transmission and maybe in the days of the 3-speed Torque-Flite. For those touting the benefits of huge mileage gains coming from running an engine through some sort of a hybrid system, in order to have the engine running at the peak of efficiency, there are much easier ways to accomplish much the same, by just adding more gear ratios to the transmission. Used to be that 3 and an over drive was something but now we have 8, 9, or even 10 speeds with a lockup torque convertor. It is very possible to have an ICE running close to peak efficiency with a common planetary transmission. Highly unlikely that you will overcome the extra wind drag that comes with an additional 5 MPH when the engine will be very close to optimal with the wide array of speeds available using today's transmissions.