I've been slowing down quite voluntarily; that's how I found that my car can usually beat 40 MPG (compared to its EPA-rated 38) at 65 MPH, and will handily whoop 45 MPG at 60 MPH.
I've bought a little car and slowed right down too. But the real savings (to be had in Australia at least) are by tail-gating road trains (really big trucks) on the highways.
In Australia there is a hugh road haul business East/West and these guys go fast (they are private contractors under the hammer). They will go up to 100mph, 160km/hr at night. I followed one (close behind)for 400km when working in the Goldfields and used 1/4 of a tank instead of a whole tank. A few stone chips in the car though.
here's a german car that gets 157mpg @ a cost of 11,000: Loremo on the Edge due out sooner than later but people are starting to pre-order.
For the past four months, I've been driving no faster than 55 MPH on my daily 23-mile round-trip commute in Austin.  The posted speed limit is 65 MPH and I drive using cruise control in the rightmost (slow) lane of a divided six-line highway.  I drive a Prius and obtain hot-weather mileage of 52 MPG with the air conditioning unit operating.  Given that Austin is a leading green U.S. city, I believe that "slow down & save" road signs would be well-received here.  A well-informed driving public would thus be even more accepting of, and aware of (very) slow drivers in the rightmost lane.
slowing down works wonders for me too ... but i've noticed that keeping speed pegged to 60mph on the open highway is actually hard work (this is sans cruise control). it's as though i've been conditioned by years of speeding to get to somewhere between 65 & 70 and maintain the range. i don't have a lead foot at all ... i'm just so used to a certain range of speed ... wild
This is why I think going back to 55 mph limit would be impossible unless it was done voluntarily - it does seem so slow with cars now built to so handily achieve and handle 70 - 80 mph.

Plus enforcement is a joke - not that I blame the cops - I would be hesitant to pull over speeders in the middle of the free-for-all that is typical of an interstate...

I've thought for a long time that there should be limiters of some sort on engines that would not allow cruising speeds over 70 mph - but then how would you be able to work in the ability to get the extra punch when you really need it (i.e. emergency situation, passing etc) ?

Still I can't see why giant pick-ups hauling trailers with boats etc etc need to be flying down the road at 80 mph.  I'm a geologist and drive a full size Ford (work truck) for field work - I've had to haul a few things and been in a couple situations that really required a bit of power - but I've been doing this about 12 years and the times I've truly needed what these trucks are capable of are very few and far between.  They are overpowered for image rather than utility - and they waste a huge amount of fuel.

I suspect a lot of those truckers are flying on No-Doze and coffee.

On some highways the newest, smoothest lane is the leftmost lane, so everyone stays left to avoid that thump-thump-thump where the concrete joints don't align anymore. During long, boring drives, I've often thought that building a left lane, or several left lanes, with bad joints and maintaining right lanes better might induce drivers to save the left lanes for quick passes. It also might slow some people down - although I've seen some pretty fast action on some really decrepit highways in Connecticut, home of the "Road Legally Closed" dodge back in the 70s.