Fuel Mileage booster’s

I am always trying new things. 25 years back, I tried the 'broquet', invented by RAF engineer Henry Broquet. [ www.broquet.co.uk ]; good for 300,000 miles. Watch out for copy cats. It was claimed to improve economy. I didn't notice any economy improvement in my GTO, but I never did any controlled type of testing either.
What I did notice: a very distinct change in combustion sound. Hard to explain, but at WOT at high rpms, the engine is definitely smoother. You can feel it more than hear it.

Thank goodness not everybody dismisses something new/different as snake oil....or we would still be using solid axles & a crank handle to start our cars...