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This presents two reasons for standard fuel gauges' imprecision:

In the sending unit, the fuel has to drop below a certain level before the float starts to drop.

When the float is near the top of the tank, the wiper on the variable resistor rests close to the grounded (negative) side, which means that the resistance is small and a relatively large amount of current passes through the sending unit back to the fuel gauge. As the level in the tank drops, the float sinks, the wiper moves, the resistance increases and the amount of current sent back to the gauge decreases.

This mechanism is one reason for the inaccuracy of fuel gauges. You may have noticed how your gauge tends to stay on full for quite a while after filling up. When your tank is full, the float is at its maximum raised position -- its upward movement is limited either by the rod it's connected to or by the top of the tank. This means that the float is submerged, and it won't start to sink until the fuel level drops to almost the bottom of the float. The needle on the gauge won't start to move until the float starts to sink.

Something similar can happen when the float nears the bottom of the tank. Often, the range of motion does not extend to the very bottom, so the float can reach the bottom of its travel while there is still fuel in the tank. This is why, on most cars, the needle goes below empty and eventually stops moving while there is still gas left in the tank.

Another possible cause of inaccuracy is the shape of the fuel tanks. Fuel tanks on cars today are made from plastic, molded to fit into very tight spaces on the cars. Often, the tank may be shaped to fit around pieces of the car body or frame. This means that when the float reaches the halfway point on the tank, there may be more or less than half of the fuel left in the tank, depending on its shape.

  • "on most cars, the needle goes below empty and eventually stops moving while there is still gas left in the tank." Really? I can't think of any cars less than 30 years old where that happens. "This means that when the float reaches the halfway point on the tank, there may be more or less than half of the fuel left in the tank" - in most modern cars the instrument displays are computer-controlled, not directly wired to the sensors, so that the shape of the tank is irrelevant. – alephzero Nov 22 '18 at 19:40
  • It has always seemed intentional to me. I believe I've seen articles pointing out that if your tank doesn't drop for a long time, psychologically you tend to believe it's a very efficient car and have higher approval. Whether that's true or not, the gauge and even the computer numbers do seem erratic/biased often (I have not driven a 2018+ car). This article from YourMechanic has some values of errors, and so may be interesting/useful to some. – JeopardyTempest Dec 23 '18 at 7:45
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Some cars have a "range to empty" shown on the dash - this is a calculation based on what the computer thinks is in the tank and the rate of fuel use based on fuel use over a period of time (1 minute, 5 or 10...).

The owner's manual will usually state something along the lines of "any fuel level indication is an estimate" and the user should make sure to fill up ahead of time...

Do not rely on the distance to empty thinking "oh it says 10km to go and it is 9km to the fuel station"...

  • FWIW my (European designed) car displays a warning light when the estimated range drops to 75 miles, and an audible warning, text message, and a continuous flashing light at 20 miles remaining. But I suppose some klutz would still manage to run out of fuel even then! – alephzero Nov 22 '18 at 19:43
  • @alephzero my car will simulate rough running when the level gets too low ie about 30 miles or so left in the tank and after that, it will stop the engine - as it is a diesel, then running dry causes lots of damage to the pump (and the pump is known to be fragile when run dry...). – Solar Mike Nov 22 '18 at 19:46

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