I admit to not knowing about hypermilling until now, and clicking the link in the article didn't load the page(my internet is a bit screwed right now), but I can't help but be dumbstruck by the shallowness of the article....
There are FAR more things that can be done to a vehicle that the author hasn't mentioned, done, or perhaps thought of. I'll list some off here in no particular order of importance:
1) Supercharging or turbocharging the engine. While this is an expensive and sometimes time consuming endeavor(I personally installed a supercharger on my 2010 dodge challenger srt8), the benefits are amazing. An engine is simply put an air pump. Doesn't matter if it runs on gasoline or diesel, the majority of what passes through the engine is air. In the case of the article, the motor he uses is gasoline powered. Gasoline burns in a concentration of 4-9%, So depending on the tuning and setup, the engine is pumping 91-96% air through it. Increasing the efficiency of the air movement adds performance and fuel economy. If the engine has more power, it doesn't need to work as hard or as long to accelerate or maintain speed. On the cheap one can look into a ram air setup. While not helping at low speeds, at cruising speed its a poor mans turbo and helps increase your efficiency. Although if you wish to simply slap on the fake hook scoop stickers, please step out in front of a moving bus before you do....
2) Changing engine components and polishing them. The air going to the motor should take as few turns as possible to reduce losses. Every bend in an air intake system reduces power. The air filter is a necessary evil, but there are higher performance ones out there available at decent prices. Not ALL filters add amazing power savings and mileage and do your homework. Next up, the intake manifold and the cylinder heads. Aluminum intake manifolds are available on the aftermarket for a large amount of vehicles. Polishing the intake and cylinder heads reduces friction of the air flowing inside them, increasing efficiency. Its not all about the amount of air being forced into the cylinder, the velocity of the air has a big impact as well. Valves in the head can be helped with a three angle valve job.
3) Exhaust. Scavenging is the term given to using the flow of exhaust creating a vacuum to draw the unspent gasses out of the cylinder. Its a common myth that you need backpressure for the vehicle to work properly. While the vehicle does require some for scavenging to work properly, too much restriction causes the engine to have to work harder to push the gasses out and drops efficiency. Equal length headers to a proper sized exhaust system with an h-pipe, or x-pipe depending on your vehicles needs can not only help it sound better(for some of us who enjoy the roar), and help with the mileage as well.
4) Weight distribution. Relocating the battery to the passenger side of the trunk to offset the weight of the driver in the front left is common in north America now, but across the pond I guess you'd put it on the other side. While carpooling is great, a lot of us have to drive alone so balancing the weight load in the car helps. Removing items in the car that you don't need to carry with you improves mileage as well.
5) Wax. Yeah, just waxing your car helps. Unless your car is brand new and has a perfect clear coat on top of the paint, you're going to have micro pits in the paint, and while they are small, they are numerous. Given the sheer amount of air rushing over the surface of the vehicle, putting a $7 bottle of wax and some elbow grease to make it nice and smooth helps reduce the drag on the vehicles surface. Also helps protect it from the road salt and washing it is a lot easier.
6) The temperature of the fuel. Yeah, lets get radicle here. If you're wanting to modify this for some fuel mileage, lets have some fun. The density of your gasoline changes with temperature. When its colder out, your gasoline is denser. Your fuel pump doesn't care about this density change, it just pumps its set amount of liters per hour and throws them down the pipe. Adjusting this can be done passively. Take a line off the engine cooling lines using a high thermal conductive material and run it along the fuel line to pre-heat your fuel to a set temperature (the engines operating temperature). Or you could design an in tank electric system to run off a thermostat and set the desired temperature yourself with testing to find the best temp. Getting more dense cool air into the engine and a set temperature and density of fuel with it can help achieve more mileage. This will need to be done with SOME consideration of risks. You don't want electrical fires in your gas tank. Heating and thereby expanding the fuel too much can reduce how much fuel you actually get and reduce power. A system where you can turn on and off the fuel line heat to run colder denser fuel for more power for say a hill or passing cars then hitting a switch and heating it for fuel mileage is another thing to try. Again, you have to consider the engine requirements to get the most out of this.
7) Using the research of race teams. Almost all of the improvements we enjoy in our modern vehicles are the results of the racing industry pushing boundaries to get all the edge they can. Exhaust headers, blowers, air foils, lightening the vehicle with fiberglass and carbon fiber components, etc. Someone else has already done this research, use it to your advantage.
Theres SO MUCH MORE that can be done, you've barely scratched the surface with this article.... AND this list was all off the top of my HEAD! I'm no automotive engineer, just a guy with a few tools and time on his hands....