Do you also remember the “test tube” catalytic converter replacement?
I remember the GT Energy Chamber which would save you several gallons per mile. Then I got some additive for the gas which saved me several more miles per gallon. Then I got some metallic pills which when dropped in the gas tank saved me more miles per gallon. Then I got a magnetic alignment kit which aligned gas molecules to burn better and saved me more gallons per mile.
I saved so much gas I had to stop every 25 miles and drain 5 gallons out of the gas tank to keep it from overflowing! ;-D
I remember people dropping the catalytic converters off so they could use Regular gas. Later when laws made that illegal people would drop off the converter and drive a rod through the innards, then would put it back on. Inspectors could not tell it had been fixed to run on Regular.