Mystery Fuel Mileage

My owners manual states the fuel capacity to be 19 gallons. I say this because I am at wits end trying to figure out the average fuel mileage of this car. Apparently doing the math (adding or subtracting from 19) doesn't work either. I don't know if vapor, air or whatever limits the fill capacity. I have recorded as low as 21 and as high as 29 miles to the gallon. I got 29 miles to the gallon during a sophmoric high speed run down Texas farm roads averaging over 95 mph for several hours. However, for the most part, I drive the car in the same manner week to week and under the same conditions. The car has met all the recommended servicing required by Hyundai and is meticulously maintained. It is a 2004 model and has a 4 cylinder instead of the more common 6. Someday I am going to stick a 2 gallon can of fuel in the car and drive it till it stops. Because of the mileage variance I am too chicken to take it past 25 miles after the light goes on. As an aside, I also fuel by mileage on trips because I don't trust the gauge or mileage I get. By the way, other than this anomaly I love this vehicle. Has anyone else out there suffered from such variance in fuel mileage with their Hyundais?

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