>So,  for me, road horsepower is the main statistic that describes what the
>vehicle is doing in the real world.

The "road hp" for the vanagon is, if I remember an early test
article correctly, about 30hp at 60mph. At a fairly low speed
like this tire friction is a major part of this (10-15 hp), and the easist
way of increasing mpg, passing power, and all the other things
you mention is to get a tire with lower rolling resistance.
Note that rolling resistance can vary by a factor of 2 between
different tires!

Unfortunately manufacturers don't publish rolling resistance
numbers. As you observe, the general public couldn't care less.
Big fat tires is the fashion and to h*ll with fuel economy,
CO2 emissons, snow and wet traction and many other things...

So how to find rolling resistance? What one want's is the requered
force to pull 1000 kg. (and then just multiply with the vehicle
weight). I don't know anybody that publishes this for different
tires. Consumer Reports publishes some relative data. In their roughly annual
tire test they compare how far different tires roll when put on
an identical car.

>More importantly some engine can produce 50 hp very economically,  but not
>produce anymore than a total of 100 hp.  Typically engines that produce two
>hundred horsepower, will be gas hogs at producing 50hp.

Throttled spark ignition (gas) engines typically have a fairly narrow 
rpm range around peak torque where they are the most efficient. 
Compression ignition (diesel) engines don't have throttling losses
at all, and are both more efficient in general (around 35% compared
to 25% for a gas engine) and retains this efficiency over a wider 
rpm and load range.

/Martin
--
Martin Jagersand                 email: jag@cs.rochester.edu
Computer Science Department             jag@cs.chalmers.se
University of Rochester 

        Slow down and visit the VW diesel Westy page:
          WWW: http://www.cs.rochester.edu/u/jag/vw
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