Has anyone else noticed that when you have a full fuel tank, the Fuel Averages always improves. After fuel is used, say down to 3/4 full, the averages began to decrease. Combined driving.

Example: Fueled up yesterday and fuel average is close to 24.5 Gal used. After using the fuel down to say 3/4's to a half, it normally settles down to about 22 or 21.5.
Running around town and highway combined.

Cheers

 

Hi Chuck,

I haven't noticed that in our car, but it's curious. It certainly does seem counterintuitive that fuel economy would be better with the greater weight of a full tank.

I assume you mean the average fuel economy as reported by the onboard computer? Remember that the computer isn't measuring true average fuel economy, it's just calculating a rolling average based on various sensor inputs, including the fuel lever sensor in the tank. If, for example, the resistance of the fuel level sensor is not exactly linear, or if the arm moves less freely at the upper end of its range, the fuel use computer might underestimate the actual fuel usage in part of the sensor's range compared to other parts of its range.

I haven't tested our X350, but in our XJ40 the onboard computer's average fuel economy is typically 2-3 mpg wrong compared to fuel use carefully calculated by miles traveled (measured by GPS) divided by gallons of fuel used (to the 100th of a gallon, as reported by our local Shell station's pumps).

Cheers,

Don