Actually, wax, the best way for you to test this is:

Cut open an aluminum can, or anything that will fit around the device and won't melt. Wrap it in insulation and put it around the device.

Fill up your tank and record mileage and fuel use every day by topping it off again each night. Keep a record of your route, traffic conditions, kilometers run and fuel needed to refill the tank to the brim. Use the same gas station each night, and the same pump. Do this for a month.

During that month, have your wife or significant other remove the device each night and then put the cover back on again, with or WITHOUT the device at random, and have her record which days you ran without the device, WITHOUT telling you. This way, you will not know when the device is attached, and you will drive the same way each day.

This way, at least, you will see if it actually does anything.

I'm betting you will see a wide variance in your consumption with NO relation to whether or not you have the device attached.