I'm not sure what you're trying to ask there. A 1% decrease in fuel usage would be a 1% decrease in fuel usage (i.e., 1 gallon per 50 miles would become .99 gallons per 50 miles).
The calculator provided in the tools section of this page might give you a lot of numbers to mess around with in order to calculate engine efficiency. Also, someone recently posted a link to the EPA's website on overall efficiencies in a car. That also might be worth your time to read.
http://ecomodder.com/forum/showthrea...epa-20754.html