I watched the Mythbuster's episode too, and they did try to account for the differences, using the same car each time, and also controlling for the added weight of the clay they used to create the dimples. I believe they were using an electronic fuel flow meter to get exact quantities of fuel used, so it should be accurate. However, I too thought the improvement sounded high, and, as mentioned above, if it were really that great, we would all be driving dimpled cars now, because the manufacturer's would love a 12% increase in mileage to meet new CAFE regs...