One of the reasons the estimates for Peak Oil are off is because oil companies only project the amount of oil they can economically recover. However, because the price of oil continues to go up they start to consider more options economical, such as tar sands. If you read literature on how these companies determine the size of their reserves and how much oil is left that can be extracted economcally, the answer is pretty much consistent at 40 years, but this measure has never been an accurate account of the total amount of oil in the ground, its just a number that makes sense for their business.