A look inside a plug-in fuel economiser for cars.

This is a generic clone of a very dubious product that goes under various names with buzzwords like “neo”, “shark” and “doctor”.
These devices claim to reduce your fuel consumption by balancing the electrical power in your vehicle. This one from ebay is quite positively a luxury version, since it does at least contain a 1000uF 25V capacitor alongside the obligatory LED that shows something’s happening.
This one contains twice as many components as needed for it’s function, with two 1K resistors in series with the LED, a bizarrely high 1.5 Megohm resistor across the LED and the capacitor just banged across positive and negative. There’s a fuse which is nice.
So basically it has a white LED that lights up under the futuristic looking blue cover. The microscopic level of supply smoothing is not going to save you fuel or increase the vehicles power.
It’s a cheap placebo. But at least it didn’t go as far as potting the “electronics” in resin like some versions (with just LEDs and a resistor) do.

ironically people who think they are getting poor fuel performance will almost inevitably think something like this has improved their mileage greatly when they actually work out the fuel consumption of their vehicle. They would get the exact same result if they worked out their fuel consumption without this plugged into their lighter socket.