Go ahead with your test as is it free. But whatever results you get ( + or -), don't tie them to this gizmo. An on-the-road fuel consumption test is at best inacurate as it is subject to so many variables: (weater, wind forces, wind direction, temperature, traffic, load, tire pressure, type of gasoline used, idle time, state of tune of the car, different driving attitude based on the mood of the driver and mayby 10 others.
If you have noticed the installation instructions, you will see they highly recommend the use of injector cleaner over 2500Km before you see any results. The cleaner itself would have a positive effect on your fuel consumption if the injectors are dirty.
For better results, I would suggest the use of this gizmo, coupled with a front-bumper-mounted magnet facing the front of the car. The magnetic attraction created by the magnet toward the cars in front of yours has also been proven very effective. However, you loose the advantages gained when you back-up.