I do about 480 miles a week highway. I was doing pretty bad around 25-26mpg for awhile. Changed my air filter, plugs and cleaned my PCV and I just got 30 on my first full tank. Usually travel 65-75 on the highway and I have a manual transmission.
One thing to keep in mind is the computer MPG reading and the actual calculated (miles / gallons) are not always equal...On my last one my computer said 31.2 but I calculated 29.6.