[redpoint5]

MPG gauges don't rely on energy content of the fuel or fuel capacity to make their calculations. Fuel injectors are controlled by the computer to open for a certain amount of time, called the duration. For each intake stroke of the engine, the injector opens for the amount of time instructed, and that combined with the fuel pressure and flow rate of the injector allows fuel consumption to be calculated continuously. Combine fuel consumption with speed, and you can then calculate miles per gallon.

If you were somehow able to get a fuel with higher energy content, the car would simply go faster for a given fuel rate. Since fuel rate and speed are the only things needed to calculate MPG, the calculation would still be correct.

Miles per Gallon is a math formula expressed as Miles driven per (per always means divided by) Gallons consumed. Since this formula doesn't include any other parameters such as energy density of the fuel, detergent amount, or anything else, we don't need that info to do the calculation.

Suplimenting your Subie's instrumentation is helpful in monitoring other efficiency parameters such as engine load and timing advance, as well as being able to read check engine codes and clear them. Nearly anything monitored by the ECU is able to be reported on a Scangauge or UltraGauge since the gauges can be customized.

I mostly monitor engine load, timing advance, and engine coolant temperature and have alarms set to warn me if anything gets too hot.