Elephant: When I fill up at the bottleneck, if it's me or someone else, you can tell me the difference
Aside from strict scientific objectivity, because indeed, nobody doubts your ability to fill up, absolutely at the same pump, etc ...
What is important in the context of a measurement campaign is that the driver has no indication of the results so as not to change anything in his driving, or, if the experimenter and the driver are the same person, they see the odometer. It's already too much! For example, using the red fuel light on my own vehicle, I can easily see if I have driven 820 or 860 km with a full tank. The ideal for testing an economizer is a taxi or express mail company or the fleet of an installation or breakdown assistance company.
It is absolutely necessary to eliminate the placebo effect or awareness of the presence of the device.