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......

On the NHW20 Prius, the generator max rpm is 10,000rpm, therefore the Prius engine won't run above 2,800rpm at 0mph.
Are you sure it is OK to run the generator at 15,000rpm on your FEH?

Ken@Japan

The chart is based on the NHW10 Prius.
Prius and FEH use the same THS technology and have the same kind of limitations.
The ICE on both Prius and FEH always spins above 42mph to protect over-spinning of the generator.

Ken@Japan

So are we all saying that if you are getting on the highway for example and trying to get up to speed that you should be running it at about 4,000 RMP's?

Thanks for all the info everyone I am learning a lot about this.

By the way my first tank of gas yeilded me 29.4 MPG. That was traveling up to Maine at 28 degrees with a slight uphill trip. The return was in about 40 degrees. Travel speed was at 70mph on the highway. So far I am cautiously happy.

That much is true.

That, for the most part, is untrue!!!!

Using commercially available scan tools I have measured generator speeds in the 12,000 RPM ballpark in the Ford Hybrid Escape. There is a 40 miles per hour EV limit, but generator speed is not the limiting factor, since the generator is at only 6000 RPM ( half of it's acceptable range ) at 40 miles per hour. A Ford service document says "brief excursions to 13,000 RPM are permissable".

My theory is, above 40 miles per hour there may not be enough available torque to start the engine in some senarios above 40 MPH. Another theory is, the quick deceleration of the generator
required as the engine starts may exceed acceptable limits for "harshness & vibration".

I do think the transmission is similar to the Prius in general, but is obviously, much more robust. -John