Some cars fuel gauges are like that, depending on the possition the sensor is in the tank. My friends sentra goes from full to medium in like 20 minutes(probably much longer but thats what it seems) but it stays at Medium forever. So, it could be vise versa for the Z because my gauge goes down quickly too. But hell im thinking way too much because in 1 mile that is almost impossible. Oh well, atleast im thinking right? :P