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MPG is all very well and good, considering that most people that are concerned about their fuel consumption tend to want to get as many miles as they can out of it before they need to fill up again. However, I was stuck in traffic recently averaging a rather measly 4.3 MPG simply due to the fact that I had the engine on whilst I was scraping the ice off the car before I left as well as being in a lot of stop-start traffic when I did, hence not accumulating many miles for the duration of time that the engine was turning over. That does not necessarily mean that the car was inefficient, just that I wasn't able to complete that many miles per gallon.

What is the best alternative metric that I can use to be able to find or calculate the actual efficiency of the engine?

Also, how can I find out what this metric is on my car? Can I do it through the OBD-II port?

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  • Note that for a more linear, traditional measurement, GPM or Gallons per Mile (or liters per km) is a better way to compare the fuel consumption of different cars. Saying "this car is 5 MPG better than the other" makes a huge difference when you're talking about 10MPG vs 15MPG, but doesn't mean much when you're comparing 55MPG with 60MPG. Comparing GPM remains consistent across the range.
    – JPhi1618
    Jan 26, 2016 at 15:07

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There is Brake-Specific Fuel Consumption

Which is a measure of how much fuel is consumed per unit energy.

Another way to look at it is the rate of fuel flow needed per unit power developed by the engine.

This information is not something accessible through OBD-II.

But it's not very useful in this case

Note that MPG remains relevant because it is a measure of how far you travel on a unit of fuel. It is a far more practical measure of how efficient a vehicle is in transporting occupants. Of course, if the vehicle is just idling in traffic, this metric will not reflect well on fuel economy.

I wouldn't consider BSFC to be a good measure of fuel economy at idle. I'd say something like litres per hour (fuel spent per unit time) is far more representative of the cost of idling.

As a plus, this is something accessible through OBD-II. Mode 01, PID 5E will give you the current fuel flow rate.

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