Slower RPM = fewer injections of fuel per minute = less fuel burned. That's a gimmie.

That's what GaryG is talking about. But lower RPM also equals less power.

There are "steps". Here are the best stair steps, if you use your foot to control:

1800 RPM
2400 RPM
3000 RPM
4000 RPM

You can run at 3500, but you get more bang for your buck going all the way to 4000.
You'll get to speed quicker, so you can back down to a lower step sooner.
-John

P.S. After 3 years of eCVT I now drive a 5-speed Insight. Wow... It's like learning to drive all over again! It is so frustrating to me when I'm in 5th and I press the go-go pedal to the floor, and nothing happens! Why is my engine not reving? Oh.... yeah..... Ops.... gotta down shift......