That's a common problem with Euro cars (have that problem on the Volvo). First time I've heard of it in a Sentra.

Is your speedo working? If it is, I suspect the Odo gears are at fault. Replacement odo gears for a Volvo costs US$50.00 online and is a DIY job (the dealer will not replace odo gear, only the whole odo assembly).

I'm not sure where you can source an odo repair kit locally. I haven't fixed the one on the Volvo yet because of the job's difficulty. Besides, true mileage can always be checked via the diagnostic computer of the dealer.

The least painful way would be to slap on a surplus cluster.