While I cant speak for throttle body spacers and cold air intakes, I do not buy into the theory that what is supplied by the manufacturer is the best and most efficient and there is absolutely no improvement to what they have supplied. Manufacturers are driven by the same principle that the aftermarket industry is driven, MONEY. They produce their vehicles to produce the most PROFIT, not the best gas mileage, longevity, serviceablity or any other principle. They also have to worry about EPA standards which may or may not be a concern of ours. In todays market, people will pay more for these improvements from the manufacturer, i.e. Toyota Prius, that they would not have 20 years ago. And MONEY is the motivator there too, the reduced fuel cost to the consumer. As this market develops, it will be interesting to watch the balance of purchase price vs. what the manufacturer will supply to help out the customers pocket book in the long haul and what the consume will pay to obtain these benefits. While TB Spacers and CAI's may not be the answer, there are things out there that DO improve some vehicle as supplied to us by the factory. Case in point was my old '78 3/4 ton Suburban 4WD. Stock it got a solid 9mpg. With the addition of headers, duals, aluminum intake and a Holley carb, the mileage increased to over 13 mpg. Interestingly enough too, after snatching the 350 out and installing a small block 400 with the same accessories, the mileage went over 14 mpg. So certainly, the manufacturers do not supply a vehicle that was "engineered to get the best it can do." Maybe it was the best it could do under the constraints of PROFIT and the EPA, but certainly not the best it was capable of. My .02.