I’m a meticulous record-keeper, particularly when it comes to either my car or my finances.

In our recent ethanol case study, Dave Stokes and I argued that the E-10 savings projections reported in the Missouri Corn Merchandising Council’s study were wrong partly because they failed to address the fuel efficiency decrease of ethanol-blended fuel that had been noted in numerous scientific studies, including one by the Environmental Protection Agency.

So, I’ve been curious to see how much the E-10 mandate has affected my car’s individual performance. After filling up my car this morning, I looked through my fuel log and made a back-of-the-envelope calculation of the difference in fuel efficiency this year.

My car has a 13-gallon tank, but I typically fill up about 12 gallons on average. In 2007, my car averaged 308 miles between fill-ups (25.67 miles/gallon). This year, my car has averaged 281 miles between fill-ups (23.42 miles/gallon). That’s a drop in fuel efficiency of 8.77 percent.

Now, admittedly, this is a little bit of an ad hoc calculation and other variables clearly impacted my car’s gas mileage. But Missouri’s E-10 mandate has obviously played some role.

So, how much has the drop in fuel efficiency cost me? Let’s say I fill up my car twice a month (24 gallons). With $4-per-gallon gas, the 8.77 percent drop in fuel efficiency will cost me nearly $100 this year.

So much for E-10 savings.