Simply put, real Formula 1 finishing order is often determined more by pit strategies then actual lap times. The same applies to our sim races and I have been the lucky recipient many times.
Formula 1 cars are extremely weight sensitive. More so then any other car that I know of. Ounces make a difference. Pounds make or break your race. 1 gallon of fuel in your car is equal to 6 pounds of added weight. That additional 6 pounds equals 0.10 added to your lap time. Carrying an additional 5 gallons of fuel (roughly 18 liters) costs you a half-second per lap. Not even tire wear matches the amount of time lost to carrying a few extra gallons of fuel. This fact has to be measured and compared to the time you would lose by making an additional pit stop.
Some tracks have very short pit lanes (Monza, Montreal) with easy entrances and exits. Minimal time is lost entering/exit the pit lane at these tracks. Total loss of track time is under 20 seconds which can easily be made up at a half-second per lap over a 60 lap race (that is 30 seconds for the math impaired). Other tracks have longer pit lanes or very slow entrances/exits (Barcelona, China). They cost you more time and may not warrant additional stops.
It is not that easy though. You have to factor in what type of track it is too. Tracks that demand a lot of accelerating or low speed cornering may works better with lighter fuel loads. Longer straights will less burst-acceleration zones may prefer heavier fuel loads and allow less pit stops. This is something that you have to determine on your own in testing. There is no rule book to lay it out for you.
The pit strategy basics
2-stopper: The standard F1 pit strategy with a compromise between speed and less time in pit lane. Probably the safest pit strategy in a full distance GP and very widely used in real F1. It also allows a lot of flexibility on WHEN those stops are going to take place which is extremely vital (more on that later).
3-stopper: For the all-out hot lapper that wants to put himself at the front of the grid. Being out front in an F1 car means more then you may think. The car's grip is determined by the air flowing over the wings and body. Having cars in front of you will disturb that air and prevent you from lapping as fast as you are capable of. The down-side is having to spend more time at a stand still in pit lane. However, at some tracks, that penalty is not as severe. As I explained above, you can easily gain a half-second per lap over the 2-stoppers and a full second (or more) per lap over the 1-stoppers. This time gained can make up for the time you lose in pit lane.
More advanced thoughts
If you are 2-stopping and your competition is on a 1-stop strategy, consider WHEN you are going to be pitting. You know the 1-stopper will be pitting at roughly half-distance. You objective would be to be to take full advantage of his slow lap times when he is at his heaviest. To do this, you need to be at your lightest fuel load for a few laps. if he pits on lap-30, you want to be pitting by lap 35. 5 complete laps at a 3 seconds gain per lap is close to the time it takes you to make a pit stop! Also consider that during the first half of the race you will probably be lapping faster then the 1-stopper and should have pulled a gap to him before your first stop. A gap of 10 seconds is all you need to win the battle (10 sec gap + 15 sec gain from overlapped pitting = 25 sec... the time for your extra pit stop).
You may think additional laps overlap would be better, but you have to factor in the fuel that it takes to run those additional laps. Trying to go 10-12 laps beyond his half-way point pit means you have to carry that much more fuel in your fuel run, at a loss of a few tenths per lap, completely canceling out any gain you may think you had.
Pulling a gap early in a race is worth more then a light fuel load late in a race and trying to chase down your competition. Pulling a gap on your competitor kills his will to fight and he will eventually settle for his position behind you. Even if he does keep his will to fight, he is the one that has to push to keep up with you. One missed brake point is all it will take and the race is yours. Also, the later in the race, the harder it is to maintain focus and concentration... for you both. Trying to hot-lap during that time can easily turn into a race ending mistake for you. Lastly, the later in a race the less cars on track. This makes it much easier to make laps around the track without traffic getting in the way. Sounds like a plus, but that same plus works for him too, making it that much more difficult to chase down your competition.
I have adopted a fairly aggressive pit strategy during S6. For qualifying I fuel my car to a 3-stop fuel load. This allows me to qualify up front and hopefully lead the race by the end of the first lap (it's been working in most cases). Now that I am in the lead, I have 10-14 laps worth of fuel to try to pull a gap. If I can pull 1 second per lap from my main rivals, I exit the pits 10-15 seconds behind them. I also plan this first stop to be made just prior to running up on lapped traffic (which usually gets heavy about lap-12 at most tracks). While my competition is in traffic, I have a 10-second gap of clean track. I may be at my heaviest fuel load, but he can not take advantage of that due to traffic. Once he pits, I easily pass him and usually have a much larger gap then I originally had. Now he is at his heaviest and I am fairly light. Again, all I need is to pull a few more seconds gap and I have the 24 seconds needed to make another stop and STAY ahead of him after exiting the pits. My 2nd stop is where I may end up running close to a 1-stop fuel load. It is late in the race, everyone is growing tired, track is empty, and all the gaps are already set. The race is effectively won by this point and there is nothing he can do to challenge it. The race was won back in qualifying, not during the race itself. Fuel load puts me up front on the grid. Fuel load allows me to pull a gap early on. Fuel load allows me to avoid lapped traffic and maintain best lap times longer then my competition. Of course, this strategy can be countered by using other strategies...