Placing your trust in the fuel gauge of a small general aviation airplane makes about as much sense as shooting a piano for its ivory. As a practical matter, fuel gauges in small GA airplanes serve one purpose: They tell you when you have a leak. That's it. After all, the federal aviation regulations require fuel gauges to read accurately only in level flight with all the usable fuel gone. The only time the gauge is required to be accurate is when it's too late for the information to be of much use to you!
That's why it's important to wean your students off their natural trust in airplane fuel gauges early in their training. In its place, try having your students make an estimate of fuel used on every flight, regardless of whether you're flying cross-country or doing local pattern work. This should be a vital part of every flight lesson.
Begin with the performance chart to get a ballpark idea of the fuel consumption. Have your students identify the start time, departure time, time to climb, time aloft, and engine stop time. Then, have them make a rough estimate of the fuel used on the flight. Compare the actual amount of fuel used with the estimate and discuss the reason for any differences between these values. On future flights, have them refine their estimates of fuel used based on these observed values.
While you may spend fewer than two or three minutes making these estimates, your students will begin to trust their judgment more than they will the fuel gauges. Remember, the float system used in many airplanes is the same system used in the modern commode. When a commode goes bad, you aren't likely to suffer a forced landing. The same can't be said for those who rely on fuel gauges to determine the amount of fuel on board their airplane.