Hi Chuck,
I haven't measured the regenerative energy efficiency but 30% counting both capture and subsequent use in drive is probably not too far off from the real world. In ordinary driving, the rates of battery charge and discharge reduce the regenerative round-trip efficiency significantly. NOTE: this will be an important aspect of the Volt if it ever arrives in the show rooms.

We typically deal with battery "C" units, the ampere hour, which for a Prius is 6.5 Ah. If regeneratively charging a battery at 1C, it would provide ~1.9 kW of braking power, about 2.6 hp. This would be painfully inadequate and limiting the battery acceleration to 6.5 A would be even worse. We're typically seeing much higher charge/discharge rates, 5-10C, and the efficiency really falls off in this range.

Fortunately hybrid vehicle efficiency, at least with the Prius, is much more than kinetic energy recovery. A better model, one that is reproducible, starts with the vehicle characteristics.

The primary determination is vehicle drag because that sets the upper limit on what can be achieved at any given speed. The product of the drag times the speed gives the power required to maintain that speed. A driver can do worse but they can't do better without making modifications to the vehicle to reduce the drag: (1) rolling and (2) aerodynamic. For example, rolling drag is helped by tires, alignment and lubricants and aerodynamic changes require shields and fairings.

The second aspect is efficient power plant operation. This means making sure the ICE and hybrid drive systems stay in the maximum efficiency range. In the case of the Prius at speeds below 42 mph, the vehicle control computers will cycle between efficient ICE operation and battery cruise without any further effort by the driver. My Prius ICE efficiency runs about 31% at speeds under 70 mph. But for efficient driving, just hold the vehicle at a steady speed on a level surface.

Near as I can tell, holding my NHW11 Prius at 15 mph on a flat surface (aka., shopping mall parking lot) gives over 100 MPG as long as one wants. I've done this three times over 30 minute intervals at temperatures as low at the mid-50s. A Ford Escape, Highlander, Camry or Lexus should, if held to the same, steady 15 mph should get outstanding MPG on a standard day.

The third aspect is route planning or 'smart highway' driving. That means avoiding braking and where possible, minimizing the rolling and aerodynamic drag. But on the commons of public roads, that takes planning.

Bob Wilson