Chevrolet just came out with their Cobalt XFE model that improved highway mileage from 33 to 36 mpg for 2008 and 37 for 2009. As I understand, they changed the camshaft profile, reprogrammed the computer, revised the transmission and final gear ratios, removed the spare tire and replaced it with a sealant kit and an inflator pump for reduced vehicle weight and also used low rolling resistance tires.
My guess is that the camshaft revision is a small portion of the 3 or 4 mpg improvement. You can take it from there.