Gas Mileage

I just purchased a 1999 Hyundai Accent from a used car dealer with 74500 miles. Based on what I have read online I was expecting to get 30 MPG. I have only been getting about 20 mpg with mostly highway driving. I have replaced the dirty air filter and VIP reccomends a tune up with a fuel system cleaning for $159. Can the need for a tune up be causing this big of a drop in gas mileage?
Thanks,
Peter