A: TACHOMETER is used to know engine RPM (Revolutions Per Minute). It has nothing to do with fuel efficiency as such. It merely tells you that how fast your engine's crank-shaft is rotating. Tachometer is meant for enthusiasts/speedsters only because they need to know how fast their car's engine is rotating. There is no such rule that you need to change gear at some particular RPM. But yes, there are limits for doing same. You should change gear where your car demands it or you feel you should do it. Some cars can run in 5th gear at very low RPMs, this is because the car is pouring very good torque at low RPMs. This is good for daily heavy-traffic driving and bad for speed-seeking guys.
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