An old pal, Don Sherman, who used to be the technical editor of Car and 
Driver magazine taught me a handy relationship that faciltates rigorous 
observations with regard to speedometer readings.  The mph times seconds 
per mile always equals 3600, so dividing your stop watch observed seconds 
between interstate highway mile markers into 3600 tells you exactly what 
your ACTUAL average speed is for that mile.
  Now lets assume your'e going along at 1000 mph.  In your head you know 
that right on every 3.6 seconds you pass a mile marker.  But how about 
the 58 seconds that my diesel Westy can maintain (in the radiator cooling 
rain)?  Pocket calculator tells without a chart or interpolation.  
  It was seeing the optimistic speedoemeter note that brought this to 
mind.  I've used it for years, and enjoyed plotting my turbo charged BMW 
diesel 6 cylinder 2.4 liter motor powered Lincoln Continental's computer 
mpg display against speed.  Using 10 mile incremets, from 48 mph to well 
above the speed limit, I verified that there are "sweet spots" where 
things work best.  I can only speculate as to why the smooth curve that 
the squared and cubed load factors predict doesn't fit worth a damn, but 
it's true.  Enventually I'll be able to report this data to you about the 
stock 1.6 liter diesel Westy.  At that 58 second or would you prefer 62 
mph cruising speed that my Westy can hold, the Lincoln gets 40 mpg with 
both air conditioning and cruise control running, its sweet spot.
  If there are other performance number collectors who'd be willing to 
share their observations based on odometer error corrected miles driven 
and brim full to brim full tank fills, I'd love to see your chart 
from say 50 mph to top speed.
  That reminds me, why does diesel fuel foam so much?  It takes several 
minutes to get an absolutely full tank, due to this frustrating foaming.
		John   
Remember the number 3600, a stop watch and a calculator.