159 miles so far, all gasoline free. I can't be of help on this question at this time.

 

Note sure exactly at that MPH but I have a ScanGauge device and can check that next month when I get the car.

Here is the general info and the answer to your last question. BTW in Mtn Mode the ICE revs higher to keep a higher SOC (state of charge) for the 'mtn'.

http://gm-volt.com/2009/10/26/chevy-volt-engine-generator-operates-between-1200-and-4000-rpm/

The Chevy Volt’s 1.4 L 4 cylinder flex fuel generator garners a lot of attention and discussion online.

It is this range-extender that make the car so unique. Although the Volt always operates as an EV, once the battery’s state of charge reaches roughly 30% this generator powers on, providing electricity to the motor. Energy is also obtained from regeneration and the battery buffer when power demands are high.

GM has yet to publicly demonstrate the car operating in this charge-sustaining mode, but most accounts indicate it is unnoticeable.
In spite of many months of rampant speculation here, GM’s lead Volt engineer Andrew Farah has finally disclosed some details about the engine’s operation. He said the generator would “operate from 1200 to 4000 RPMs and from a 30% to 100% load.”
Further explanation comes from Volt Powertrain Engineer Alex Cattelan:

“We don’t keep it at a fixed RPM, we have a window of operation that is optimized. We have been able to optimize the engine for a window of efficiency but it is still best to change your power and torque levels within that window as the customer torque request varies.”

“We don’t want to always be operating at one state because really you may be putting too much energy into the battery or drawing too much energy out of the battery. It is still good to vary that engine power and torque. Not to follow exactly what the accelerator pedal does, but to optimize efficiency.”

Finally Volt vehicle line director Tony Posawatz explains it this way:
“In charge sustaining or range extender mode, the Volt will not follow the throttle position. The challenge is to select the right operating points (RPMs) that are 1.) efficient, 2.) pleasing to the driver, and 3.) meet regulatory requirements.”
And he teases us, “we are about ready to expose people to this experience.”

 

Just to directly and clearly answer the original question, yes the RPM varies. It would be more efficient to operate at a single RPM point, but it's too "weird" for the driver so they vary the RPM to sort of match your car speed and throttle position. The Volt revs the engine up to whatever level it thinks it can get away with psychologically. It almost always shuts off at low speeds, meaning that as you roll to a stop and then start moving again, it's pure electric.

If you stomp on the accelerator, the engine will rev up to higher RPMs within a few seconds -- but you'll get the electric acceleration right way (aka the EV grin).

I vaguely recall that there are 4 or 5 discrete RPM operating points, and that 4800 RPM is the highest. WopOnTour may know.

 

The Volt revs the engine up to whatever level it thinks it can get away with psychologically.

Yup. Mostly sits at one of several plateaus, running only as fast as it figures it can get away with masked (mostly) by the road sounds. If you sneak up on it, and ask for a *lot* of power quickly, you can get it to rev several plateaus past what it would normally like to do.

A little weird, but kinda fun IMHO.