There isn't an additive out there where I've noticed a difference in regards to fuel economy. Sometimes there's a bit less smoke, and an interesting odor emanates out the twin-tailpipes. Perhaps a bit more power off the line too, for the first 50 miles. But nothing lasting.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)