Originally Posted By: Arney

At a reasonable 250mA current draw, that single D cell will last 16 hours. But the AA will last 9 hours, so the D cell provides less than twice the runtime even though it contains 6 or 7 times the total energy as the AA cell. Now which one looks more efficient?

Duracell appears to rate their Procell D (PC1300) at around 60 hour at 250 mAh at 70F, though its hard to figure out their non-specs.

Duracell Data sheets

The real advantage to a D comes at much lower discharge rates, 10 mAh or less. A 3D direct-drive LED will be pretty dim at 5 mAh, but it will be a long, long time between "dim" and "dead".