I want to get a 2004 acura rl but i don't like the gas mileage?

I would like to increase the gas mileage for the city up to 21 instead of 16..the gas mileage i would like 2 improve up to 27 instead of 22. I heard theres a chemical you can put in your car to boast the gas mileage up more. Another thing I want to do is to add another exhaust pipe on the left side...is that possible or what? Thanks and if so how much would that cost?