For a number of years I worked for Champion Sparkplug up in Toledo, in the Engineering Test Lab. At one time we bought every item listed in aftermarket that claimed to "improve" gas milage, etc. etc. These were put on a fleet of some 38 cars, owned by Champion, including a Caddy, Buick, several Chryslers, and about a dozen private cars owned by engineers and some of us in the lab. Each car was dyno tuned on our equipment and set at factory spec. The cars were driven for 35 days by us. The gas milage in "normal" driving, city mixed with expressway driving was recorded. Each car was then checked on a wheel dyno for specific output at set rpm. Then over a week end the fleet was taken out to a test track owned by Spicer, and loaned to us for this specific test. Each car was driven by professioinal drivers for 200 miles (100 mi. with the gadget, 100 mi. with standard factory gear), with milage and speeds recorded. In NO case was there any noted improvement using the aftermarket milage inhancing equipment. In some 71% of the cars there was a noted and provable reduction of some 2 to 7 mpg. using the add-ons. The best way, (but not the most fun)to improve milage, is to keep your car tuned up properly, replace the belts & air cleaner, as needed, keep your radiator and cooling system (pressure cap, hoses, water pump and thermostat) in good condition and at the proper rating. That done, reduce your speed, and use the speed control as much as possible. (It keeps the engine at a set speed). Sorry guys there ain't no such thing as a "free lunch." You want better milage, more HP, add a freer breathing exhaust, (reduce back-pressure) change the chip in the OBDC etc. None of which is cheap. With the federal mandated emission gear in place, other than 'blue printing' an engine or spending massive hunks of bucks, it cann't be done.
Sam Feder '01 V70 2.4T, '91 '940GLE, '87 300ZX, '48 M.G. TC, (money poor but car rich!!!)