My trip computer has two readouts for fuel consumption, described in the book as Instantaneous fuel consumption and Average fuel consumption.
The behaviour you describe exactly matches my instantaneous fuel consumption, It shows the actual consumption at that specific instant and it is constantly changing. The reason that it often shows 99.9mpg isthat this is the highest figure the readout can display. My previous car, A Skoda Octavia would show instant readings to three digits and would frequently show consumption of 200 or 300mpg.
The average figure shows the average consumption since the computer was last set and is thus an average of all the instantaneous figures you mentiom
Edited by: dethadol