As a side note: I worked in the factory that make Chevy Tahoes when they started making 2 mode hybrids. The hybrids didn't make more highway mpg, but they improved city milage from 16 mpg to 20 mpg. One of the problems is that doesn't sound like much of an improvement, but it's actually 1 1/4 gallons saved per 100 miles.
100 / 10 = 10
100 / 12 = 8⅓
10 - 8⅓ = 1⅔
100 / 30 = 3⅓
100 / 60 = 1⅔
3⅓ - 1⅔ = 1⅔