I don't think that most folks here are postulating that the manufacturers build the most efficient vehicle. What I think has been said repeatedly is that the manufacturers, being driven by sales and profit, build the most efficient vehicle they can FOR THE MONEY. So, it is not unexpected that one can spend a fair chunk of money and get ones vehicle to perform better -- at least, under some circumstances. However, in the case of BT spacers, and some of the goofy aftermarket air intakes that one sees advertised, it would have cost the manufacturer little, if any, extra manufacturing costs to make their vehicle with such items installed OEM if they truely worked. If, for example, some inexpensive item like that raised the power output of the engine 15%, most manufacturers would jump at the change to advertise that their vehicle put out 310 hp rather than 270 hp. Conversely, if they could install some relatively inexpensive modification and be able to raise the MPG figure one or two MPG, they'd jump at the chance. For example, I have a Chevy 5.3 V8 in my SUV. By using various sensor inputs, and programming a few computer circuits, they are able to engineer their "active fuel management" system that did result in significantly raising the freeway mileage figures. That was a big selling point that yielded them more sales as well as more CAFE goodie points.