> Petrol stores 50 times more energy by weight, but it needs to be exploded. A doughnut stores around 15 times, but it needs to be digested.

You left out a very important fact: Combustion requires external oxygen, which is not counted in the denominator when energy-by-mass is typically considered.

Ah that's a really good point, so it reduces the x times for both petrol and doughnuts. I'll recalculate thank you!