What do you mean by gain you much? gaining fuel capacity isnt the point of this. the point of it is if you forget to fill up or start to run low on gas. ( A rather be safe than sorry scenario) And so everyone knows let me explain how the reserve works on it. Ok you start out with a full tank of gas, with the gas turned onto the "on" position. After riding for a while the gas level gets below the golden part on the long tube wich then dosent allow gas to reach your carb wich will cause your bike to shut off. Then you know that you need to fill up with more gas. So then you swich to "reserve" wich opens up the short tube at the bottom of the tank and allows you to use the rest of the gas in the tank to get back to somewhere where you can fill back up.