or at a slope or more than u2212 11 a d units 20 sec e g between u2212 11 and u2212 13 a d units 20 sec depending upon system requirements and preferences a second point along the difference curve the fuel level signal 110 is a zero point 116 i e the point at which the pressure curve 108 crosses the temperature curve 106 in a d units in the illustrated embodiment the inventors of the present application have observed that the zero point 116 normally is at about the midpoint of the portion of the downward sloping difference curve the fuel level signal 110 that extends between the knee 114 and the end point or tank empty point 118 of the difference curve the zero point 116 also serves as a reference point for predicting how far down the difference curve the fuel level signal 110 will extend beyond the knee 114 i e the difference curve generally extends as many a d units beyond the zero point 116 to reach the tank empty point 118 as the difference curve extends from the knee 114 to reach the zero point 116 fuel level states exhibited by a fuel level meter or display are determined by using the knee 114 the zero point 116 and the tank empty point 118 to calculate set points along the downward sloping portion of the difference curve the fuel level signal 110 the set points define trip locations along the difference curve at which the display is switched to a higher or lower fuel level indication in currently used.