[ngrimm]

Thank you for pointing me to the "tools", the aero, rolling resistance calculator helped me figure it out. I started with the sample car at .22% engine effic. and got 52.53 mpg @ 55 mph and changing the effic. to .23% gave 55.02 mpg or 2.4 mpg improvement or 4.56%. Then I added higher weight, frontal area, and CD to bring the car to 37.56 mpg at .22% engine effic. a 1% engine effic improvement gave 39.27mpg which is also a 4.5% mpg improvement.