You're right Tim, provided the effort is the same.If a runner with an oxygen uptake of 4 litres a minute, converts to cycling, his oxygen uptake on the bike wouldn't be quite as high. This is because his quadricep muscles wouldn't be as developed.
After time though his Quadricep muscles will "catch up" with the other muscles, enabling the athlete to make use of the full 4 litres of oxygen.
Remember in the transition from running to cycling, the oxygen carrying capacity of the blood is unchanged. What is different though is the usage of this oxygen in the muscles(Quadriceps).
There is a way of working out calorie consumption, during endurance exercise.I'm going to include it, in the book I'm writing.
Regards