Charge and Save modes both use exactly the same logic with how to run the engine, so there will be little difference in revs between Save and Charge on the same piece of road. As mentioned, Save turns off the ICE once the preset SOC is achieved, whereas Charge just runs the ICE till the SOC is nearly full.
As jaapv alluded to, the engine will run differently depending on load and road speed. In Series mode (below ~40mph), the ICE will run solely as a generator with any consequent losses. The ICE can run at any speed depending on the load. In Parallel mode (above ~40mph) the ICE will be connected directly to the front wheels, so will run like a normal car in "top" gear, with the ICE revs mirroring road speed. However, the car will try to make the ICE generate more power than is required, and it will feed this extra power into the generator to recharge the battery. It will try to run the ICE at its most efficient power for the given revs, so improving fuel economy.
So the car will use petrol more efficiently when in Parallel mode. Therefore on a long journey, you should try to use EV (and not run the ICE) when travelling below 40mph, and only use the ICE above 40mph. Also high power demands on the battery are bad for degradation, so try not use EV on the motorway. If you know you've got a long, slow drive at the end of a long drive, by all means use Charge to refill the battery, as it should be more efficient to do that and then use EV in town. Always try to arrive at your next charge point with an empty battery, as that will improve fuel economy.
So I switch Save on as soon as I get on the motorway (once the SOC has dropped a bit) and try not to use battery power at high speeds to preserve the battery. As soon as we slow down, I'll switch off Save.
Of course, you could just leave it in 'D' and let the car do the work. I'm not convinced that the economy would be much worse and it's a lot less hassle
