I have gotten down into the 13 range with my 2004 LSV8. That was when I had a 10 mile commute, all surface streets with a couple good long hills thrown in.
My current all freeway, 100% at 70+mph in the am and about 70% 60+mph in the pm for 30 miles each way gets me right around 20mpg.
Street driving really kills the mileage. I can show 21mpg on the computer and as soon as I get on the street it starts to drop and drop.
This mileage is a bit disappointing since my much heavier low tech 96 Chebby SS got almost the same mileage as the LS.
Jim Henderson