The present day piston engine instruments used in the typical general aviation airplane are not precision laboratory instruments. We exclude the turbine and jet powered aircraft from this discussion and will consider only piston engines, recognizing that the more expensive pressurized twin engine models may also be exceptions.
Nevertheless, the purpose of this brief presentation is a practical approach to interpreting the readings of your engine instruments in accomplishing a safe and efficient flight. If, for example, you were to observe an irregular reading of one engine instrument, it calls for a cross-check on all other instruments, and not relying on one instrument as a basis for a decision affecting flight.
Since the engine is dependent on fuel, we consider the gasoline gage as a related engine instrument. If pilots are going to attempt to stretch their flight range close to limits, they should be aware of the errors in the gages vs. the actual usable fuel. Some modern single engine aircraft have had the gas gage show several gallons remaining, when in reality the tank was empty. Others have indicated a specific number of gallons when filled, but actually the tank held several gallons less than indicated.
Therefore, in planning for each flight, remember that general aviation engine instruments are not precision laboratory types, so cross-check, and give yourself an extra margin for safety.