I believe wind resistance would be a bit less than that, or about 1.6 times (assuming resistance proportoinal to the square) 55 -> 78mph would be double.

Anyway, here are my top five tips to save fuel without reducing overall average speed, most of them have to do with city driving (whcih I actually don't do very often, my MPG would probably be better if I did more of it):

1) Set tire pressure to the highest level you're comfortable with (as far as ride/noise level), not exceeding max cold sidewall.

Not only does this reduce rolling resistance noticably (I actually coast faster down known stretches of hills at 44psi as opposed to 30), it improves handling and hydroplaning performance as well. Drawbacks are a bumpier and noisier ride.

2) Pay attention to traffic light timing, coast from as far back as possible if you suspect the light will be red when you arrive.

This one is quite broad for a single point, and probably takes the most practice of anything, though it can be huge for city driving efficiency. The key though in the city is to coast to lights if you suspect they will be red by the time you get there. This will use no extra fuel, save your brakes, and take no extra time as you'd need to stop anyway.

3) If a ine of backed-up cars is formed, approach it at just the right speed so that you reach it just as it disperses

This is applicable both in city traffic, such as a backed up line of cars at a signal that has just turned green, or in highway traffic congestion conditoins. It also has the benefit of improving the fuel efficiency of drivers following behind you in a sort of chain reaction, multiplying the environmental benefit of using less fuel, all without decreasing average driving speed.

Another more "advanced" application of this has to do with approaching red lights, and guessing when they will turn green. This can be done by analzying the cross-traffic on the intersecting road -- if lots of cars are moving through, it will probably stay red a while, so you're best off getting in as much regenerative braking as possible. If they are beginning to stop, and there are no cars in line to turn left, the signal should turn green soon, in which case coasting is a better option, so you'll already be moving when the signal turns green.

4) Balance air conditioning load

When air conditioning use is necessary, run it at a high temperature setting such as 82 degrees on flat stretches, run max AC on the coldest setting when going downhill, switch AC off when going uphill.

This basically balances out the load of the Air conditoiner. I personally wish Honda would have designed the "econ" mode to actually do this for me, rather than just be an on/off switch for auto-stop, but on long highway trips, it's not too much work to do this when coming to hills.

5) Drive with load over hills

This works both on highway and city rdiving as well, and is similar to the AC cylcing method, or more evently spreading out the work the engine has to do, so it can stay in an efficient powerband more of the time. This principle is acutally the basis of hybrid technolgy itself -- using the electric motor to add power rather than requiring the engine to run outside its efficient RPM range.

Finally, the reason I emphasize driving without reducing average speed is that I prefer to go with the ambient flow of traffic in order to reduce interactions such as lane changes, and speed changes. These are also all essentially "free" savings as there is no added time cost, simply a change in style.