I was stuck in slow traffic, and watching my fuel (thanks to IKE and long gas lines) so I was using the the gas to try to get the CVT to shift into a higher range when the MPG jumped to 100+ I looked down and had 3 bars of ASSIST I was doing MPH, this kept up for about 15 min. before I had to brake, the highest speed I got was 17 MPH! I'm going to have to find out what the limits are unless some one out there can fill me in. I thought only the Priris ran on battery alone.

 

You can't start out with assist only but once you get up to speed the car can motivate solely on electricity up to ~35mph on flat ground. You can do this at freeway speeds if you're on a downhill. The only problem is that it drains the battery pretty quickly and once you get below 5 bars of SoC, you loose the abilty to assist-glide. I'm surprised you were able to keep it up for 15 minutes--were you on a downhill?

 

15 minutes seems VERY long for full electric! Perhaps you were bouncing in and out of it? By the way, 10 MPH is the magic minimum to engage full electric. If you slow below 10, it will start the ICE. Also, you can do from one to four bars of assist in this "mode", depending upon pedal pressure. Above four and it starts the ICE. Someone once claimed they were able to get five bars in electric... but I've never been able to do it!

 

Five bars is possible, but insanely difficult. I also was a disbeliever, even with a photo from someone in the Pittsburgh area to prove it, until I also got it about two months ago on a Columbus > Cleveland run (that averaged 74mpg) while going downhill at about 60mph.

 

Ronnie,
The engine has 3 modes or 'stages' of valve control.

1 is regular burn for when you are accelerating or regular driving.
2 is 'lean burn' where you are just burning enough fuel to maintain speed.
3 is 'no burn' or EV Glide where the engine isn't burning any fuel. Also referred to as coasting.

Maximizing the time you are in 2 or 3 will get you maximum gas mileage.

In your example, you were in 3, but you were using assist to maintain speed. I'm surprised you were able to do it that long without totally zapping your battery charge.

This video from Honda explains how they work:
http://world.honda.com/HDTV/news/2005-4050705b/

I have a scanguage and using the throttle position (TPS) and ignition timing (IGN) displays, I have figured out which mode I'm in.

If you don't have a scanguage, you can tell when you are in 3 by the iFCD showing 'maxed out' at 100+mpg and you can feel a little bump when the valves close. You get into it by taking your foot off of the gas pedal and then you will see the green bars. If you gently press the pedal until there are no green bars and no white bars, you are gliding without using any fuel or battery power. You will lose speed, so this is good for slowing down at stoplights and in traffic. If you have a scanguage, you will see the TPS drop to 12 and the IGN will be zero.

To get into lean burn mode, get up to speed, say 35 MPH, then slowly let your foot off the pedal until the iFCD goes from ~50-60 MPG up to 80-90+ MPG. You should be able to maintain speed on a flat road. If you have a scanguage, the TPS will be 14-15 and the IGN will be advanced ~35.

When I'm in heavy traffic, every morning and afternoon, to stop and go I repeat this procedure:
Wait for the car in front of me to move ahead 20+ feet or so and then let off the brake and gently accelerate up to 15 MPH. then I take my foot off the gas for a second to get into glide. I immediately press the pedal enough to cancel the green bars/charge and glide down to about 10 MPH where the engine wants to start up again. I take my foot off the gas pedal and when I get down to 8 MPH I gently touch the brake pedal so that when the MPH drops to 7, the car will auto stop. You can coast a long way after the engine autostops, as long as you keep your foot gently touching the brake pedal.

I usually get between 60 and 63 MPG on my 25 mile commute each way. I don't use AC in the morning and try not to use it in the afternoon. I park under a tree at work, so that helps!

And as always, YMMV!