Tracking MPG versus Average MPH on a per tank basis

[DoctorM]

Vavg versus mpg

I use my on-board computer to track my average speed (Vavg) for each re-fueling. The computer tracks every second that my engine is running (until I reset it). It also calculates my average speed since last reset.

Vavg is just the mileage since last refueing, divided by the hours the engine was running since last re-fueling. It is a much more accurate and refined notion than City versus Highway driving percentage (which is just a guess (a "WAG")

I then plot the coordinates(Vavg, mpg) for each re-fueling, as on the above graph. This one graph tells me whether my driving habits need reviewing or not. As long as I am within the two orange lines, I have done all I can reasonably do to be efficient.

The upper orange line (upper bounds) is derived from just two points - the EPA estimated highway miles/gallon (assumed at at 55mph average speed), and the EPA estimated city miles/gallon (assumed at 20 mph average speed).
The shape of the upper bounds I estimate from my engineering knowledge.

The lower orange line (lower bounds) I just estimate at 10% less than the upper bounds -- which accounts for driving conditions we usually cannot control, such as driving uphill, strong headwinds, outside air temperature, humidity, etc., contaminants in fuels, etc.

I have posted this and some of my engineering analysis on fuel / driving efficiency on my web site at http://www.transcend7.com/fuel_efficiency.htm