From: indira@ghandi.net   
      
   Snag wrote:   
      
   >    You might see a slight decrease in gas mileage and power due to the   
   > slightly retarded spark timing . Modern electronic controls will   
   > optimize it all .   
      
   Years ago I ran a test on a vehicle which stated it needed premium and I   
   couldn't "measure" any difference whatsoever in gas mileage, even as I   
   agree fully with your premise that there "should" be a "slight decrease" in   
   gas mileage.   
      
   I think the reason I couldn't measure it was that the measurement system we   
   typically use isn't precise enough because the decrease is too slight. We   
   measure by the typical method of what I refer to as "gallons used" tests.   
   a. Fill the tank   
   b. Drive normally until you need to fill the tank again (note the miles)   
   c. Fill the tank again (note the number of gallons used & divide miles/gal)   
      
   In my experience, this "gallons used" method isn't precise enough to   
   measure the "slight decrease" you'd expect from lower octane rated fuel.   
      
   When they really test gas mileage, they put a known WEIGHT of fuel in a   
   plastic bag attached to the fuel lines, and run that and then weigh the   
   fuel after a set number of miles. Our gallons-used test is less precise.   
      
   Also, we don't normalize the type of driving, where in general, it's my   
   understanding, that the only time the "wrong" gas should knock is in when   
   the spark is advanced which doesn't happen all that often in normal   
   driving.   
      
   Sure, if you race everyone out of a stoplight, you'll be advanced, but most   
   normal driving doesn't use spark advance all that much (although all are   
   advanced - but I'm referring to extra degrees of spark advance situations).   
      
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