Watching the Tour of Flanders today reminded me of an article on the Salvation site regarding horsepower and the depth of the fuel tank. An excerpt:

"Do you know why you train? Have you ever defined your goals, precisely? True fitness, which is different from the appearance of fitness, generally comes down to the power plant. Do you need a big and pretty engine, a powerful engine, or perhaps a big gas tank? The answer depends entirely on the objective.

If success depends on measured distribution of effort, efficient fuel assimilation and expenditure, and on the certainty that pressing the pedal five hours into it will cause the engine to respond with something better than a cough, then it's the capacity of the tank rather than the size or appearance of the engine that matters. Long endurance efforts universally require one to carry his/her engine so power-to-weight ratio is equally important: increase power and endurance without gaining size or compromising oxygen efficiency."