Tank Chart vs Gauge: Which Number Do You Trust?

They answer differently because they measure differently. A float gauge is a mechanical estimate with real-world slop — ±5% is typical when healthy, worse when the float sticks or the linkage wears. A chart reading converts a direct depth measurement through exact geometry; its only errors are how well you read the stick and how well the chart matches your tank.

Order of trust: a metered delivery ticket beats everything; a careful stick reading plus the correct chart is next; the float gauge is a convenience indicator for "roughly where am I", not a number to schedule deliveries against in January.

When chart and gauge disagree by more than a needle's width, the gauge is almost always the liar — but check your side first: right chart for the tank size and orientation (275 vertical and horizontal differ massively per inch), stick held vertical, measured from the true tank bottom.

The professional habit is calibration: stick the tank and note the gauge at the same moment, a few times across a season. You'll learn your gauge's personal error curve — many read accurate at half and optimistic near empty, which is precisely where it hurts. Charts for every standard tank are in the library.