Trip speed calculator reset?
 

I have a 2009 Montero Sport (3.1 engine). The on-board computer that tells you your average speed, mpg, miles left based on fuel etc is acting weird. The average speed never shows more that 12-14 mph. It used to work now doesn't. Is there a way to test or reset it - or a part I should look at replacing? I used to have a Subaru and it had a black box that read inertia to feed the computer the speed you were traveling.... does my Montero?

Also is that information cumulative as long as its not re-set or do I have to reset it before each trip I take?

TIA