Consumer reports had a report on various fuel saving devices a few months ago. They found that SPECIFICALLY that the TORNADO actually seemes to WASTE a little more gas than it saved if you had a late model vehicle of 4 years or older!
They also found that almost NO fuel saving device or additive that they detected was worth the cash. Most of the fuel additive were found to be INDEED FUEL! A little was added but not enough to pay $1.25 for a bottle of fuel additive that really cost about 30 cents or so.
I would recommend that you change you oil and filter a little more often than usual and POSSIBLY to change your fuel filter if you have an older car of 3 years or older!
New cars SHOULD not use any fuel saving devices so stated by Consumer Reports as it may cause you problems with the system.
Personally, I dont use any fuel additives, but do change the oil filter more than most and I change my air filter more than most would