A trip between star systems could easily use up 500 units. How can I increase the maximum fuel capacity?
There is no way to increase the Normandy's fuel reserves in ME3. However if you travel around the map you might get a light ping when you hit a certain area (similar noise to when you approach a planet). Actively pinging in close proximity to this hidden planet will reveal a destroyed fuel depot which will give you a one time fuel refill of X amount of fuel. There's usually one in each system. However revisiting this destroyed depot will not refuel you.
WARNING: locating this hidden fuel depot may attract Reapers.