So far we have recorded between 15 and 25% improvement in economy but some drivers do better – largely because of their adaptation to “downspeeding” a phenomenon especially for manual transmission drivers  who exploit the increased power at low revs to drive in higher gears at the same speeds they used to in lower gears because the engine response and torque feels the same – while the less power strokes/mile ensures each individual one is more powerful which is achieved due to increased thermal efficiency.