A 15 mpg vehicle uses 6667 gal/100,000 miles.
A 18 mpg vehicle uses 5555 gal/100,000 miles.- It saves 1111 gallons.

A 30 mpg vehicle uses 3333gal/100,000 miles.
A 40 mpg vehicle uses 2500 gal/100,000 miles. It saves 833 gallons.We really should use 36 mpg instead of 40 mpg, but since 40 mpg was stated,I'll use it.

The smaller % savings on the gas guzzler is much more important. GM is making the right choice in one sense. However, a 3 mpg increase just doesn't "look" as big as a 6 mpg increase. I use 6 mpg rather than 10 mpg because the percentage improvement of different sized vehicles should be kind of close, if the same hybrid system is used.Luck,Charlie