Ditto. The main reason I keep a spreadsheet of MPG is to gauge for any problems. In my old fiesta, I kept records of fuel use and found the car seemed to lack power at one point. This was backed by a drop in MPG which then lead to further investigations and a repair.
Ditto with the MK5. The car didn't feel down on power much but at one point the MPG dropped below expected for my style of driving. It turned out to be a faulty coil pack. The drop in power wasn't immediately noticeable and it was only through MPG logs that I found it out.