Did you ever have a fuel gauge that was accurate? Same here. They either read full for days, then take a nose dive to a quarter tank, or wander about the dial hoping to be right at least some of the time. Either way, it's dangerous when you don't know if there's enough fuel to get home. So we decided to look into the causes for inaccuracies in fuel-monitoring systems. Is it the senders, the gauges, or something else?
To conduct our test, we got a stock rotomolded polyethylene permanent-mount belly tank from Moeller Marine Products, one of the country's largest suppliers of OEM and aftermarket fuel tanks. It's a typical 36-gallon model that you might find in a stern drive or outboard boat-rectangular on top, coming to a shallow V at the bottom. The first thing that became obvious was that "empty" is a relative concept. A typical pickup fitting is suspended about 1" over the bottom of the tank. The senders, no matter which type, are usually in line with that fitting. That means a tank will always have a small amount of fuel that can't be accessed. This is done so the gunk that settles to the bottom doesn't get sucked into the engine. Depending on the size of the tank, there could be quite a few gallons that can't be used. A rectangular tank lets you easily figure how much is left. If the tank is 10" deep, that unused 1" is 10 percent of the capacity. When the tank's shape is odd, so are the computations for full and empty. Because the bottom of a belly tank is V-shaped, the inaccessible fuel is less than that in a rectangular one of the same capacity. This V-shape gives your engine access to more fuel, but if the sender or gauge hasn't been configured for an irregular-shaped tank, the readings will be off.