Most efficient cruiser

[Insp3ct0r4t0r]

Figured this would be as good a place to ask as any, after spending a few hours trawling through the fuelly data. I'm in the market for a new used car. It's going to be used for my daily commute of around 100 miles each way, and practically all at 70 to 80 mph (both my home and work will be only a minute or so from the motorway and traffic is rarely heavy on this route). I want to make my purchase decision with fuel economy as the number one priority, as fuel will be by far my largest expense.

Anyone can tell me that if I want to spend less on fuel, I should just reduce my average speed to 60 mph. But let's say that for me, the extra time I would save every day by maintaining 80 mph is worth it. So which cars have the best MPG at this speed, compared to other cars?

My main problem is that most of the 'real world' economy figures that are available don't distinguish between the different types of car use very well (Fuelly is no exception). So while the average MPG for something might look like best in class, I've no way of knowing whether this still holds true at predominantly highway speeds.

Any suggestions that might match these criteria? I also don't want something too new, so that the depreciation doesn't exceed my fuel costs anyway.