Well the LS1 that came out in 1997 it didn't use atkinson cycle and it was 36% efficient at 2000 rpm under load. Combine that with the fact that all of the LS cars from 97 to now use the same 0.50 final drive ratio and only turn 1500 rpm on the highway and pretty much all of them will get 30 mpg or more at 70 mph if you try. Honestly if you can stay out of first and follow skip shift at a steady speed they get great gas mileage.
Consider at the time the Honda insight was only 38.5% thermal efficiency and was pretty much then the benchmark.
Hell my 2010 Prius runs atkinson and only makes a third of the horsepower and still only gets 41 on the interstate at 70 mph.
EPA ratings are weird because I feel like hybrids are cheating, and normal cars will always get a pinch better performance than their rating.
http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf