[Fat Charlie]

Keeping the innards clean is good- I throw a bottle of fuel system whatnot in a few times a year. I don't think of it as gaining a percentage of efficiency, though. Cleaning the fuel system recovers performance that the car had when it was built but has lost over time by not being kept clean. I expect I'd see an improvement if I used a cleaner without ever having done it before, but I'd still have to call that moving back to stock performance.

I'll stand by my position that 30 more miles a tankful is irrelevant, though. Tank distance is based on things unrelated to the car's actual performance. Personally, I don't notice even a 1 mpg jump when I add a bottle because I've been doing it and for mileage purposes I count the 4 ounces as an additional 0.031 gallons of gas. I don't factor in the price difference in my log, though. I'm buying it for cleaning, not propulsion.