How does barometric pressure affect instrument readings?

Barometric pressure has a significant influence on altitude readings in aviation instruments. Specifically, altimeters are calibrated based on standard atmospheric conditions, which include a specific barometric pressure. When actual barometric pressure deviates from this standard, which can occur due to weather changes, the altimeter may indicate an altitude that differs from the true altitude. Fluctuations in barometric pressure can cause the altimeter to read higher or lower than the aircraft's actual altitude, leading to potential navigation errors or safety risks.

Thus, understanding and accounting for changing barometric pressure is critical for pilots to ensure accurate altitude readings and maintain safe flight operations. This is why the notion that fluctuations can lead to incorrect altitude readings is the correct answer. The other options either state that barometric pressure has no effect or inaccurately attribute its impact to areas like speed measurements or fuel efficiency, which are not directly influenced by barometric variations in the same way that altitude measurements are.