These do NOT work.
A local news-station (Seattle) did a report regarding these. In their un-scientific test, they lost about %1 economy with the device attached.
Also, if this item actually "Increase your gas mileage by up to 27%" why doesn't the manufacturers put this item on all their cars.
Auto-makers are spending millions per yer trying to develop more fuel-efficient cars that meet EPA requirements.....if such a device existed, they'd use it.
Compare this item to diet-pills.... there's usually a note that says something like "...pills taken while eating a balanced low-calorie meal plan". Heck, junk the pills and follow the diet and you'll lose weight. With this in mind, maybe the %27 fuel savings includes a note like; "..after attaching, follow the following fuel saving tips....gentle on the gas-pedal, accelerate slowly, don't use AC, etc."
Don't waste your money. Don't waste your time.