Good answers guys!

One more on this comment. In my Civic, the hybrid (elctric motor) helps under acceleration, regardless of speed. So when I need to speed up on the highway, I get help. The boost I get on the highway is the same boost I get in town. Also, because I have an electric motor, I get away with a smaller enginer (1.3L). That smaller engine will burn less gas than it's 1.8 or 2L counterparts at highway crusing speeds (assuming equal RPM). I think the technology pays off at any speed.

*** Retired after 65,000 outstanding miles ***

I believe the system is called full hybrid system, which includes the Ford Escape Hybrid, both contribute power as needed at all times.

First 4WD Hypermiler

Have you read the FEH FAQ?

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the Prius (and FEH?) 'full hybrids' cannot use the electric motor >41mph or so. RPM limitations on the electric motor.

Let me comment some...

On the flat highway, you're right that the battery does not work a lot, however the MG1 and the MG2 are still working hard.
They work as an electric torque converter. The engine runs at an ideal power, then the torque is distributed to drive the wheel directly and to drive MG1 generating some electric, then it goes to MG2 to drive the wheel.
So, my opinion is the Prius HSD applies on the highway too.

Please note that the EPA highway mileage on Prius is 51MPG and Echo is 42MPG.
They are using the same engine block, but there is more than 20% difference.
Of course, they are using different engine, atkinson/otto, and body, however I believe the HSD technology is the main factor of the difference.

Ken@Japan

That's not quite correct. The reason the 40-41mph limit exists is because when the ICE is off, the sun gear needs to spin backwards, and it's limited to 10,000 RPM and that's what the speed limit works out to, above that speed the ICE needs to spin, even if it's not generating power in order to protect the small motor generator from overspin. The large motor-generator is directly connected to the output of the 'transmission' and it's top RPM is linked to the car's top speed, so it's able to supply torque at any speed, including and especially at high speed. When the ICE is off, the most limiting factor is the amount of power the battery can produce, and it's usually somewhere between 20-30 HP, but once the ICE turns on it generates as much power as the large motor-generator requires for quick acceleration.

2001 Prius Photos       2005 Prius Photos
'97 Prius Simulation Prius inner workings

Schwa is right, AFAIK.

ICE spinning in a gliding situation costs about 1 kW. When you consider that total energy demands of the Prius travelling at ~ 40 miles per hour is about 4 kW, that extra 1 or 5 MPH over 40 is costing you 20% more fuel.

R2-E2, 2G Prius.
Highway/City/Husband/Wife MPG: 56.5, as of 12/2005, 26K miles

Jac Nasser, Ford President: "We are planning to launch a hybrid version of
this car [P2000] within this year [1998]. We will also make FCEV available in
2004."

ahh. thank you for the explanation- not being an owner, I haven't delved far into the details of HSD operation. Sorry for spreading misinfomation.

Remember that all cars have batteries, not just hybrids, and all cars' batteries die and need to be replaced eventually. Hybrid batteries are merely bigger.