What could result from a broken speedometer gear tooth?

A broken speedometer gear tooth can lead to erratic speedometer readings because the tooth is responsible for driving the speedometer mechanism that translates the rotation of the drive gear into speed readings. When there is a broken tooth, the gear cannot consistently engage properly, leading to fluctuations in the speedometer's reported speed. This lack of consistent engagement means that the speedometer may jump around or provide readings that do not accurately reflect the vehicle's actual speed, resulting in unpredictable and unreliable diagnostics for the driver.

In terms of other options, complete speedometer failure could occur under more severe conditions, such as a total loss of the speedometer gear, but is not the most immediate consequence of a single tooth failure. Inaccurate fuel gauge readings are unrelated to the speedometer gear and typically stem from issues within the fuel sending unit or associated wiring. Noise in transmission might arise from many factors such as bearing wear or other mechanical issues, rather than specifically from a broken speedometer gear tooth. Thus, the broken gear tooth specifically impacts the speedometer operation directly, making erratic readings the most accurate consequence.