Pain and Stress-induced Tachycardia: The patient has recently undergone a traumatic event (hit by a car) and has a tibia fracture, which is a significant source of pain and stress. This can lead to a sympathetic response, causing tachycardia. The administration of 4L of oxygen may also indicate some degree of respiratory distress or anxiety, further contributing to tachycardia.

Hypovolemia or Blood Loss: Despite the absence of obvious pulmonary issues, the patient could be experiencing internal bleeding or hypovolemia due to the trauma, leading to tachycardia as a compensatory mechanism.
Anxiety or Panic Attack: The traumatic experience could trigger an anxiety or panic response, manifesting as tachycardia, especially in the context of receiving high-flow oxygen which might not be fully justified by the CT scan findings.
Fat Embolism Syndrome: Although less common, fat embolism syndrome (FES) can occur after long bone fractures like a tibia fracture. Early signs might include tachycardia, and it's crucial to monitor for other symptoms such as hypoxia, neurological changes, or a petechial rash.

Cardiac Contusion: Direct trauma to the chest can cause a cardiac contusion, which might not be immediately apparent on a CT scan but could lead to arrhythmias, including tachycardia.
Tension Pneumothorax: Although the CT scan does not show effusion or atelectasis, a tension pneumothorax can develop rapidly and is a life-threatening condition that requires immediate intervention. The absence of findings on a CT scan does not rule out this possibility entirely, especially if the scan was done before the condition worsened.
Sepsis: Early signs of sepsis can be non-specific, including tachycardia. Given the traumatic nature of the injury and potential for open fractures, there's a risk of infection that could lead to sepsis.

Adrenal Insufficiency: This could be considered in the context of severe trauma, especially if the patient has a pre-existing condition affecting adrenal function. However, it's less likely without other supporting signs or symptoms.
Thyroid Storm: Although rare, thyroid storm could present with tachycardia among other symptoms. It would be an unusual complication in this scenario without a known history of thyroid disease.