Turning low cadence does not automatically conserve energy, it just switches where the load is placed on your body.

You turn big gears and you'll deal more with muscle fatigue/ Lactic buildup from the anerobic stress, you spin higher cadence and it'll put more emphasis on the aerobic system where your legs will feel fine but your heart and breathing will be stressed.
You need to find the good point in the middle where anerobic and aerobic are stressed equally in most cases. This changes though as you tire in different ways. Hills, downhills etc also require different strategies.

If you merely upshift for lower cadence you lose speed unless you push harder - load is on legs
If you downshift to increase cadence you may also lose speed unless you spin faster - load on lungs and heart.

This is simplifying it a bit but is more accurate than the car analogy.