John and others,

Measuring total drag vs speed is an excellent idea. I can't see
why the method John suggests (towing with a dynamometer (*)) would
be illegal in states that allow rope towing. However, the point
that "drafting" will influence the measured values is very
valid.

Another way of calculating drag is by coasting to a stop. Accelerate to
a fixed speed and coast to a stop while measuring speed vs time.

Total drag in Watts: P(v) = mv(dv/dt),

where m = vehicle mass in kg
      v = speed in m/s
      dv/dt = (de)-acceleration in m/s^2

Practically speed vs time could be recorded by videotaping the
speedometer during the coasting and then analyzing the data on
a home vcr with an accurate second (or frame) counter. Eg record
at what second the speed falls to 70,65,60,55,50,55,... etc,
convert all units to metric and plug in to above formula.

Ideally the speedo values should be calibrated like John suggests.
The same method can be used to determine engine horse power.
First do the drag measurement. Then videtape speedo while
accelerating. Engine power = Paccel + Pdrag (as above)

Cheers,
Martin

(*) A big fish scale might work fine
--
Martin Jagersand                 email: jag@cs.yale.edu
Computer Science Department             jag@cs.rochester.edu
Yale University

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