Yeah, and it'd be real convenient when you get to an amount you haven't measured before.

BTW, since you're being annoyingly pedantic, six inches of a yardstick that measures a quarter of an inch by an inch would displace about 0.0065 gallons, which is far less than the precision you'd be able to get out of a pump. In addition, you're not taking into account the fuel you'd burn driving to a gas station. Or maybe you meant that you should call a truck to tow you to a gas station.

In all honesty, the levelness of the tank is likely to cause as much error as non-cylindrical abnormalities.