My suggestions :
- Tyres as inflated as sidewall max less 5psi at first. This helps reduce drag without affecting grip - in fact grip may improve. Try it though, your car may be different and it is your responsibility to be safe - e.g. wet weather makes roads 50% as grippy as dry. If you are happy go to max.
- Less use of cruise - it maintains a set speed no matter what, and maybe you don't need to - see below.
- Driving with load, or rather driving to an MPG. Set yourself a target MPG and try to stay over it. Put AVG together with MPG on your scangauge - if you keep MPG > AVG then you are improving.
- Combine this with a minimum safe speed so you don't get in people's way or where you can pace / follow the trucks in the slow lane.
- At the same time don't panic if your instant MPG goes lower as you will hit a peak on any journey and then it will be hard to improve. In these circumstances try to maintain MPG as high as possible.
- Don't be afraid to use topology and traffic. For example in rolling country use the downhill sections to gain speed to use on the next incline, or use the decline to EOC. In traffic coast when you can and leave a gap in front so you can cruise if the line stops. Forget fretting about people pushing in - who cares, it costs them more in $s than you, and it makes them 0% better than you overall
- Check if your vehicle EOCs OK - i.e. if you go downhill in gear does your fuel shut off - does your MPG go to infinity... (or 999.99). Most FI engines shut off fuel when decelerating in gear (i.e. on overrun) but will still idle if cruising out of gear. You need to decide when and where to use which technique - on downhills use a high gear but remember the compression in your engine will slow you down, on the flat use neutral as you coast longer.
- If you are using an Auto then out of gear is probably your best option - check your car is OK to coast out of gear or can lock up in top.
FWIW and IMHO Subarus are not the most FE friendly vehicles. For a start there is the extra weight and drag of the AWD system and secondly I don't think the flat four is a really great engine for FE.
Even the Diesel one in Europe which has the latest tech - CR injection, variable Geometry turbo - does no better than any other CR direct injection Diesel even of a few years ago, although IMHO again Japanese Diesels are a generation behind those of Europe.
Add to that the oil starvation issue (the top half of the flat cylinders is left dry when the oil drains to the lower half when the engine is stopped) - TBH I would avoid them.
Just my thoughts, but good luck.