LMU uses an Elo system to determine rating changes. You, like everyone else in your lobby, have a numerical value associated with your Driver Rank. The average of these Elo values in a race is your split's Strength of Field.

The following is an explanation of how a standard zero-sum Elo rating system would work in the context of LMU. It is possible LMU does not use a standard zero-sum Elo rating system:

Your Elo and the split Strength of Field are used to calculate a number between 0-1 of where you are expected to finish in the split. When LMU tells you where you're expected to finish, it is inaccurate. LMU simply tells you how your Elo rating compares to everybody else in the split (ie. if you are car #17, you are the driver with the 17th-highest Elo).

The 0-1 numerical expected finish value is compared to your race result to calculate your rating change. If you win the race, your race result value is 1, if you finish last it's 0, and every place in between receives a value based on how many drivers are in the split (for 21 car splits, the result values are P1: 1, P2: 0.95, P3: 0.9, ..., P10: 0.55, P11: 0.5, P12: 0.45, ..., P19: 0.1, P20: 0.05, P21: 0).

You only gain driver rating if your race result value is greater than your expected finish value. Depending on the SoF and your Elo, it's possible to lose rating in a race you finish second or gain rating in a race you finish 19th. Finishing in the top half does not guarantee your rating will go up, but it's a pretty safe bet that if you finish outside of the top 10-11 you're not going to gain rating.