Stop responding with unnecessary posts, the problem is not with the code but solving how to use serial read at the same time as using the MPU6050 without getting FIFO overflow. An other solution may be to connect the MPU6050 to another Arduino which could communicate through serial communication with the arduino which steers the motors. But does the serial.read also cause problem for the loop-time (time for it to go through the loop) and then also cause problem with PID?