We sell a ton of these. As far as power I have no idea, but most people get a little bit of mileage increase out of them. They work best on TBI engines and carbs, but gains on MPFI engines are also possible. We also sell a device called the Tornado that works well on carb and TBI engines. One guy put a Tornado on his mid 80's Chevy dually with 454 and went from 8.5 mpg on a 300 mile trip to almost 10.