Although it might seem strange, the habits of teams are quite sticky. They tend to do similar things each year. Most people will run three fuel levels: a lower fuel load for performance, a medium fuel load for the bulk of their work, and full tanks for their race simulations. For each team, these three levels are different, but, weirdly, they don’t vary much from year to year. Our opening guesses will assume that each team will do the same as they did the year before.
If you have a set of test times for the grid, and you have a set of opening assumptions (guesses) for which one of these three fuel loads each car has run, then you can convert the raw leaderboard into a pecking order. Of course, this first estimate is very inaccurate. The resulting pecking order is largely a reflection of the quality of the opening round of guesses – If you thought a team was on, say, 50kg, when, in reality they were on 100kg, then you will seriously underestimate their pace.
However, as the runs keep coming and the days unfold, this estimate is progressively refined using a simple method to keep adjusting the lower bound of the fuel that must be in the car. We do this by counting the laps. If a car does a 10 lap run, then we know that a lap consumes (say) 1.7kg, then there must have been at least 17kg of fuel in the car to do that run. In reality, teams do not like to run down to fumes in normal testing, so it is more likely that there was at least 27kg of fuel in the car. If our prior assumption was that this team normally runs 50kg for general testing, then we will adjust our initial guess to say that the car was likely to have somewhere between 27kg and 50kg of fuel in the car. Once a team has done a load of running, this estimate gets surprisingly accurate.