Can someone explain why you get better MPG in...

Because, generally, it's more efficent to go slower. With non-hybrids, the stop and go cancels the benefit of going slower in the city. With the Prius, you get rid of some of that extra waste and the efficiency of going slower comes through.

 

Plus, at city driving the ICE may not be on as the demand is lower.

 

You are right! If you look at the method used by hypermilers (pulse and glide), they aviod using the battery and got more than 100 MPG. Check it out at http://www.toyota.com/html/hybridsynergyvi...l/marathon.html.

 

Two words - AIR RESISTANCE.

Air resistance increases exponentially with velocity (speed). The amount of energy required to keep a car moving at 65MPH is extreme - compared to the amount of energy required to keep the car moving at 20MPG. At 65MPH, much, much more energy is required to keep the car at 65MPH, and all this energy comes from only one source - your gas tank. The effect increases dramatically as speed increases, resulting in lower, and lower MPG the faster you go. (Someone posted a nice little chart somewhere).

In the city, you are correct that every cycle of speed -> regenerative breaking -> accelerate from battery -> speed looses energy. (Of course, that's still free energy compared to a car without regenerative breaking). However that loss of energy pales in comparison to the energy required to push air out of the way at highway speeds.

-Ken

 

Yes... And I did it for 50 miles.. between 35 and 43.. got a little over 80 MPG. But I think it would be higher if the city speed limit was 25.. I think that was the average speed of the marathoners.. and the got 110 MPG.

Scott

 

42 mph barrier.
http://priuschat.com/index.php?showtopic=1...id=210092&st=26

Pulse and Glide.
http://hybridcars.about.com/od/ownership/a/pulseandglide.htm

Ken@Japan

 

In the city, less air resistance at lower speeds, but more energy wasted braking
On the higher speed highway, lets say no energy wasted braking, but more energy spent against the higher air resistance.

Which is better overall ?

Depends ..
How fast on the highway ?
How much braking ?

The Prius wastes less energy braking (70 - 80% of a regular car, because some of the kinetic energy is sent to the battery for later reuse), but these same general considerations hold, and account for most of the answer you are seeking. The remainder of your answer has to do with engine efficiency which plays a larger part in sorting this question out in regular cars, and less in the Pruis because of its design that minimizes poor engine performance efficiency wise in both the city and highway.

Pulse & Glide is a technique to run the engine at > 95% of practical maximum efficiency the entire trip, rather than 80 - 90% that the Prius achieves left to it's own devices.

This question *really* bugged me before I learned 'the way of the Prius'. Now it seems obvious