1.Do gas additives do much more than make money for the company? 2.Will hotter plugs increase milage? I'm told that our modern computerized cars will sense the hotter spark & feed less gas to the engine for the same amount of power. 3.Do narrower tires increase milage? Or just make the ride & handling worse? 4.Are the 'high milage' oils worth a flip? I have 120K on my Silverado & change oil & filter every 3-5,000 miles:will I gain anything?