new '05 outback sedan

Hi... i just bought an '05 Outback 3.0R Sedan and it has 10K miles on it. I love the car, but the gas mileage seems low. The computer is telling me i'm getting around 18mpg.... but the sticker says 19 - 25mpg... any ideas? I just took it on a 200mi roadtrip and best i got for all highway was 21mpg and that was doing like 70mph

-Tom