rmhockman, cruising at 55-60 is the Z's sweet spot. Using the "real time" mileage monitor on the computer, I know my mpg rating is highest at 55-65mph, usually around 30mpg. its the stop and go traffic that really kills the mileage, so if there's a lot of city driving, you're going to get horrible mileage.
consider yourself lucky to have little traffic
