As others have said your test really has nothing to do with efficiency, as your heart rate is a byproduct not actually the thing that matters, per se. It has been well established that picking a cadence involves a tradeoffs between aerobic vs muscular capacity. Lower cadences work your muscles more and high cadences work your hear more. That's why there is the saying that if your legs are tired, go to a higher cadence, and if you are out of breath, decrease your cadence.

Also it's worth mentioning that trainers do not approximate the road well because they spin down too quicky without a massive flywheel.