The tank holds 13.2 gallons (as claimed by Nissan) I have put 13.5 in there once before so I can tell you that the number Nissan claims may not be exact.
If your dizzy is leaking bad enough, it can inhibit spark production, so in an odd way, it can negatively affect your mileage. Any water in the ignition system will throw all sorts of kinks in the system.
Certain brands of fuel most definitely affect how a car runs. That top tier gasoline place claims that Turkey Hill is a good gasoline to use. Maybe it is, but my car ran a tad bit worse with that stuff compared to Exxon, Hess, Shell. I use Shell and Hess almost exclusively these days. Some people say that Hess is bad, but I have not had a bad experience with Hess, not once. And I run 28* of timing (3* advanced over a 25* base, damn near everyone else on the entire boards use a 15* base idle ignition)
Replace some important stuff, monitor your fuel mileage and fill the tank up all the way. It is truly hurting you because you are allowing fuel to change into a vapor by letting all the air stay in the tank rather than be replaced by fuel. If you really want weight savings, get rid of the stock tank, and get one that holds five gallons.