What determines when drivers pit? Fuel or Tires?

I was watching in dismay as on the 2nd to last lap both of my drivers pitted while fighting for position well above where they qualified.

As I looked over the data post race I could not see any reason as to why they were pitted early on the first planned stop and then again at race end. They had enough fuel to complete one more lap (which would have been enough) and the soft tires were at 54% both times.

So what determines when my Tech. Director pits my drivers? I was always under the assumption it was based on fuel load, asiI've had tires lower than 50% on occasions.

Any ideas or tips?

1 Don't believe anything your Tech Director tells you.
2 The level of your drivers will have an impact on their decision making, so they may occasionally pit on the wrong lap & even when you have really good drivers, they might still pit on the wrong lap.
3. If you want to be as certain as you can be that they will not pit early my suggestion would be to carry atleast 1 litre too many in each stint.
4. I am sure others can add to this list

Let me know how you get on Adam

All of the above is true. Unfortunately, glory is paid with attendance and attention.

Yes, it appears the tech directors are always pretty confident the cars will run on fumes...I was burned in my first two races by that and I swore never again.

My conundrum was that as I figured my drivers were burning through about 3.8L/lap and they both pitted with 6.9 and 6.8L in the tank on both occassions.  Softs were at 54% on both occassions as well so I have no idea what the hell happened.  Cost me at least 1.1m credits and some pride.

Fuel determins their pitting.

If tires reach to 0%, they will puncture and your lap time will be extremely longer.

Remember also, that the fuel counter can easily be at the end of the pitlane, which triggers a report of the current fuel in the car, suffice to say, you could actually have only 3L in the tank when you pit.