When we burn fuel in our engines, almost all of the energy created goes to one of three places. One of these is creating work at the propeller, which is of course the point of the whole thing. Secondly, heat energy is transferred to the cylinders, and thirdly, heat energy is discharged out of the tail pipes. Unfortunately, the waste heat of #'s 2 and 3 comprises the majority of the energy expended. Only about 20% of the combustion energy actually gets to the propeller!
Alex calculates that you save $4 to $6 per hour with proper mixture management. Here's the article.