Calculation aid here:

http://wahiduddin.net/calc/calc_hp_dp.htm

Alistair

'82 westy, diesel converted to gas in '94
http://www.members.shaw.ca/albell/

On 6-Apr-06, at 10:27 PM, John Reddick wrote:

The air is thinner at higher elevations.  A Vanagon, or anything else,
at 70 mph at 6000' elevation, has less total drag than the same
vahicle at
sea level going the same 70 mph. It therefore requires less power, and
gasoline, to go the same speed.  That is why 747's cruise at 35,000
feet,
instead of 200 feet.

      John
      VANARU.COM