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Your description of the Volt's 4 modes is incorrect. There is no need to "change to accommodate speeds above 70 mph". Your "Low Speed EV" mode (Ring Gear locked to case) is capable of reaching top speed and is in fact used whenever high power is needed. The second mode is used to increase efficiency, by using the smaller generator/motor to spin the ring gear in order to reduce the main motor RPM down into it's most efficient range, the "sweet spot"if you will. This can occur at speeds down as low as 38 MPH, whenever average power requirements are below the maximum that the smaller generator/motor can produce. The 2 gas modes are just these 2 modes with the engine running and locked to the generator. It is actually incorrect to say that the gas engine drives the wheels directly, since the car will not even move unless the main electric motor is supplying torque, it is correct to say that the gas engine _contributes_ to the torque when in the two motor mode, which adds to the efficiency gains realized by bringing the electric motors into the sweet spot, by utilizing it's torque to counter the main motor directly, whilst simultaneously spinning the generator...