I have heard that 60mph is actually more efficient than other speeds too, though I am not an engineer or mechanic so only have this to go on. Generally in terms of efficiency you do obviously get more miles to the gallon if you keep at this speed rather than going faster. However, as other editors comment, this depends so much on the kind of car and the size of the engine.
My understanding is that 60 mph is the most fuel-efficient speed, with the cruise control set if possible to maintain it. Obviously, that assumes road conditions and traffic allowing for that speed, which is not always the case. After extremely high winds a couple days ago, one news station reported that cars traveling into the wind to cover a story got 18 mpg on the way to the story and 38 mpg on the return trip!
There is an ideal speed range that can give you the best mileage for every car. This lies in the 40 - 60 mph region for most cars. Your aim should be to go at the lowest speed possible in the highest gear. This will vary for different car shapes and how heavy the car is.
To increase efficiency try to choose a smooth surface to drive on, minimize the use of air conditioning and if possible incorporate methods by which energy can be extracted when the brakes are used.
I've always read what's already been stated: that around 60-70 is most efficient, but I often define efficiency as "getting somewhere as quickly as possible." :-) Of course, police sometimes define "efficiency" as ticketing as many speeders per hour as possible.
I have heard that at highway speeds, you will get better gas mileage when you are closer to the 55-60 MPH range than if you speed even only 10 MPH faster, around the 70 mark. Around where I live, 70 MPH could still leave you in most people's dust, but I like to drive where I can get some decent bang for my buck, especially considering how much it costs to fill up these days. I am also remembering seeing an interest show one night about people who are super-crazy about getting the best mileage and plan trips accordingly. Full stops and acceleration from a stop sign or light are huge drains on gas. So are left turns where a stop or significant slow down are more likely. These people plan routes with the fewest stops, more right turns than lefts, and other factors like hills and gusty wind areas. It's all too extreme for me, but it was thought provoking.
From what I have heard and experienced, the slower you drive, the better the fuel mileage. That isn't to say you need to drive 35 in a 55, but if you keep your toes out of the carburetor, and have a light touch on the gas, you will probably experience your best gas mileage.
I really don't think that you can say that there is any one speed that is most efficient. As the previous post says, it depends on the shape of the car. But it depends even more upon the way the engine is made and how the gearing is. This will determine the speed at which the engine does the least work for the most forward motion.
This is difficult question to answer, because each car is different. So, what you would have to do is to read the manual and see what is the most effective speed limit. Also there are other variables that you need to consider as well - highway and city driving, for example. Another variable is the shape of the car. With this stated, most people say that 40 - 60 is the sweet spot for the best mileage.
It is a very interesting technical question. The efficiency of the car in terms of miles per gallon are dependent on may factors and to numerate, I may say:
1. The engine design and the fuel used. This gives the thermal efficiency which is low at very low engine rpm but is almost steady for a wide range of speed and lowers down again at very high speed.
2. The engine transmission i.e. gearing. It is the mechanical efficiency of the engine, generally low in the lower gears and goes on increasing till you reach the overdrive where it lowers again.
3. The type of tyres used, inflation pressure and road surface. The radial tyres have more grip and hence more friction with the road. Similarly the lower inflation pressure gives a more comfortable ride but increases the friction with the road hence reducing efficiency of the car. A rough road will also reduce efficiency due to drag.
4. The car body profile. The car profile determines the resistance of car with air due to drag and frontal thrust. More the speed - more the air resistance and lower the efficiency. It is a major factor as the air resistance varies directly with the square of the car speed.
Hence for different cars and different road conditions, the efficiency will vary at the same speed. With slightly higher inflation air pressure, good car profile, smooth road and top gear operation, one may expect best mileage in the speed range of 50-60 mph.
I agree with pohnpei, there is no real determined speed for efficiency. However, if you plan to save fuel, driving at slower, consistent speeds can give you more for your dollar.