The generator varies its speed, depending on load.
I think that if the battery charger is simply set to max, and the generator is fired up anytime after the car has driven just a bit, the charger will pull as many amps as it is allowed to.
That should make the generator run pretty much full out, which I would imagine would be peak efficiency for the unit.
Here's a photo of the generator. It is sort of half taken apart, the sheet metal and cowl removed for access to parts for testing and troubleshooting. I also have the official mechanics repair guide for it borrowed from the friend I got the generator from.