From Fellrnr.com, Running tips
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In an ideal world, we’d be able to easily measure our
running economy and see if things are improving. If we could measure our breath, find out how much O<sub>2</sub> we consumed and how much CO<sub>2</sub> we produce, we’d know how much energy we burned (and from fat or carbohydrate). Sadly, this is not practical, so the best measure we have of energy consumption is our heart rate. This is far from perfect, as heart rate can vary for other reasons besides supplying O<sub>2</sub> for energy production. However, I believe it is a useful approximation.
Assuming you know the distance you ran, your average heart rate and the time it took, you can calculate your efficiency. If you know your resting heart rate, enter that as well to optimize the calculation.