I've spent a few years chasing fuel economy via HHO but was always irked by the fact that I couldn't prove (to myself) that improvements were directly attributable to the HHO. At the moment, I'm trying to eliminate that uncertainty by establishing the best result I can get without HHO, then I'll know that any further improvements are the real deal.
Hi Pete - Welcome to the Forum - Interesting post in many ways.
As people become aware of their need to reduce their fuel they do change their driving habits in one way or many ways. Unscrupulous businesses offering magical pills to reduce fuel use played on this and all their devote folowers swore as to the effectiveness. They always failed on professional analysis. I have to be fair here, there were several that did work until fuel management computers and Oxygen sensors made them ineffective in modern cars.
Re HHO making a definable difference there is a simple check I have done - compare the amount of unburned hydrocarbons in the exhaust from with and without HHO. Less to zero hydrocarbons in the exhaust will indicate that more of the fuel is being burned in the cylinder being converted into mechanical energy.
I have done a considerable amount of testing of many different fuel saving methods.
Though not a long distance field test I often try a set distance to determine effectiveness of different systems and products.
I fill my tank to near overflowing, get on a highway and drive at 100kph for 100 kilometers, turn around and come back and refill at the same fuel station with the same pump. I have done this on standard unleaded, fuel saving products, octane boosting products, HHO and acetone. All gave me savings and were not influenced by changed driving habits as the speed was constant.
To verify my results I organised a prolonged field test with the owner of a business that makes efies - If you have not already seen it, have a look at http://reduceyourfuelbill.com.au/forum/index.php?topic=76.0
There is one really good result from changing your driving habits and it is not just saving fuel. I have a v6 Statesman and the mechanic says the motor is as tight as when it was new.