The results of the Swiss Harmony CarTuner test in detail:

On the first outward journey to Edinburgh we drove 653 km and consumed 34.77 litres of diesel. That corresponds to a consumption of 5.3 l/100 km.

On the way back to London it was 661 km and we filled up with 39.67 litres. That corresponds to exactly 6 l/100 km.

The second tour to Edinburgh with the CarTuner fitted gave a consumption of 4.1 l/100 km. That corresponds to a saving of 22 %.

On the way back to London with the CarTuner fitted we consumed 33.66 litres, that is 5.1 l/100 km. The saving was 15 %.

Here you can see the data at a glance:

Without CarTuner With CarTunerSaving in %
From London to Edinburgh (406 miles) 5.3 l/100 km 4.1 l/100 km 22 %
From Edinburgh to London (414 miles) 6.0 l/100 km 5.1 l/100 km 15 %