you clearly have something defective. it could be a sensor, or the BCM, or any combination of things. the MPG will fluctuate over time, and changes based on a number of things and depending on your driving style, so thats nothing new, but the display showing 99.x MPG is not kosher.

no trouble lamps correct? maybe worth a check at the obd port if you see any errors/faults.
when it involves software, it may be your best bet to just have nissan diagnose, so you are not chasing your tail.

so, questions.
its used, did you buy it private party or a dealer? if the latter, was there some warranty on it? did you ask the seller about this issue?

do you have the maintenance records on the car? any electrical modifications done to it as far as you know?

 

I bought from dealer, and the car was previously owned by a rental company. I think it is still under factory warranty as it is barely under 36K miles (it is 34.4K miles). Can I use factory warranty for this problem if so how?

 

yes take it back to the dealer and show them this. that's not normal.

 

Thanks marctronixx, scheduled an appointment with dealer for next Wednesday, will update this post later.

 

Hit 'Reset' on the screen from the first image.

 

thats great. please keep up posted! this is an interesting problem...

 

Dealer did full computer reset which apperantly can be done only at dealership, and no way to do it at home. After that the glitch was gone. This is just a software glitch.