Fuel Mileage booster’s

Turbo's can increase efficiency, but if you drive it to use the extra hp you get from adding one you won't get better mileage, gotta keep your foot out of it. Superchargers can too, but usually you lose out at cruise because you have to drive the supercharger. So as a power adder they're good but for efficiency not as good as a turbo. Also depends on your engine, higher compression raises combustion efficiency, but you have to not detonate and you have to not drive it like you stole it with the extra power. When I had my SRT4 (2.4L turbo) I could get like 29 mpg on the freeway/highway driving for efficiency. But around town with my foot in it and the boost gauge pegged it would go down to like 16 mpg.

All the other mileage boosters are a scam. If you pour it in the tank, put in in the fuel line or stuff it in the air box, it's total snake oil. A ECU tuner could in theory improve efficiency, but if you have a modern car with a factory tune and no major modifications it's not gonna do crap. They're already tuned for the best efficiency that won't screw up something else.

Keep your tire pressure at spec and check your tire pressures often, and keep your air filter clean and change it often. K&N style filters need to be clean and oiled properly, if they're dirty or over-oiled they'll be worse than a clean paper filter. Depending on the car some of the cold air intakes can help, but it depends on the factory intake design. If the factory intake is restrictive the cold air intake can help. If the factory intake is designed well, not so much.