I did a bit of homework, couldn't find anything in detail to explain thier methods or techniques which is really annoying. I might email Honda UK and ask for further details, worth a shot. They may have had VW drivers behind the wheel, so might have cheated
One technique many hypermilers use is what some term as "pulse and glide." That is, they will accelerate up to a certain speed and then shut off the engine and/or take it out of gear while gliding down to a certain speed and then repeat in sort of a rowing pattern going from point A to point B. I believe this particular technique works best in somewhat hilly terraine. If this is done in traffic, it tends to irritate other drivers for obvious reasons.
I wonder if they include the cost of new starters and clutches in their calculations of long-term economy.
I'm familiar with that one. I do a modified form in my Prius. I don't get near 100mpg of course although I did get 77.1mpg yesterday. Sadly it was only a 2 mile drive not a several hundred mile average but I'll still take it.
I always thinks it's best to remain at a constant speed for as long as possible. I can see how coasting will use little to no fuel, especially down hill, but having to accelerate again will use a lot of fuel again. I don't think the techniques used here would have put any extra strain on any components, if anything, less stress would have been caused, as driving efficiently requires much more gentle methods than those used normally.
The pulse and glide or coast can work because engines are oversized for the car to provide adequate perform for the less common times with higher load is needed, like passing and climbing a hill. Most engines will be less thermally efficient at the lower loads used for cruising.
The idea is to pulse under medium to heavy acceleration in order to have the engine running at its most efficient for output. It isn't the most efficient for covering distance, but the following coast will use very little to no fuel, so the fuel economy for the entire cycle will be high.
I have found it may not be as effective on freeways where the high speeds mean more air drag, and that slows the coast too quickly. Hybrids can pull it off by using the battery charge and motors to keep it up for a longer distance.
It can be done in traffic without being a pest, but the gains won't be as big since that requires shorter coasts and less speed loss.
I don't know if the information can be found for "car" engines but for Cummins/DD/Cat etc. you can get the information on output levels and power curves and the rpm range of optimum efficiency. For my Cummins I believe it was 1400-1800. If that information could be obtained it would likely provide the best baseline to pulse and glide between X and Y mph based on rpm range for that engine. Gear choice would be whichever gear allows operating in that rpm range and the speed limit / traffic / etc..
Might give it a go on my next trip, I'm due a fuel up at the weekend, and a food shop! Thing is, on these rural roads, people like to speed, so drifting between 40 and 60 will no doubt upset a few people. I'll only try it if the roads are quiet...