Hey, for a highway commute I definitely recommend a grill block, air dam, and tire spats, they can be done pretty easily, and are a great way to "get your feet wet" into ecomodding. All of the mods I just listed can be done as professionally, and stealthy as yoi desire.
For the grill block, you can block from the backside of the grill, and paint the blocking material black, and it will basically be invisible. That's what I have done on my 94 Civic, no one has even noticed that it's blocked.
Side note: A cross thread, is a tight thread.
As was mentioned though, the driver is a huge part in the fuel efficiency of the vehicle, and slowing down on the highway will have a significant effect.