Earlier this year, I had my alternator and "electrical load detector" replaced because the voltage going to my battery was badly out of whack. (I've read that this is a common problem with 2002 - 2003 era Honda Civics).

Since then, I had been closely monitoring my battery voltage so I could be sure that the problem really was fixed. After a while, I noticed that, in certain situations, the battery voltage would drop from around 14.4 to 12.5 volts.

I figured dropping down to 12.5 volts was "bad", so I kept the fan running to increase the electrical load and bring the battery voltage back up around 14 volts.

Not wanting to deal with the inconvenience of bringing my car in for service, I did this for some number of months until my state inspection was due and I needed an oil change.

While my car was in the shop, the mechanic called me up and asked me to describe my battery voltage problem in detail. I told him all about when it drops and how I can make it go back up to 14 if I keep the fan running.

And that's when I found out that the voltage drop was completely normal and was actually a fuel-saving feature of the vehicle. So, basically, for a number of months, during which I took several long trips, I was burning extra fuel needlessly because I misunderstood what my car was doing.

Since I've become aware of this, I've seen my fuel economy improve by about 5% and I've broken my previous single-trip fuel economy record.

Previously, the best fuel economy I had seen for a single trip was about 56 MPG (coasting was involved). My new best is about 59.9 MPG (for the same ~3 mile trip).

While I was checking my new record on my ScanGauge, I was amazed at how fast my average MPG dropped while I was idling. It literally only took about two seconds to drop from 60 MPG to about 58 MPG. Though, on my 12 mile commute, I only lose about 0.1 MPG per second while idling.