You might measure the
discharging amperage of the N7 while running these apps that you use when you driving, but without the charger connected.
Battery Monitor Widget will show you the history of amperage by time in a graph.
Edit:
Your N7 won't take all of this 2.1A from the charger.
Phones/tablets limit the amount of amperage needed.
So the charger's ability of amperage could be bigger than the phone's/tablet's needing, but should not less.
Smart phones limit to 1A and I guess the N7 might limit to 1.5A.
This amperage is divided in the battery's consume to be charged and -on battery bypasses- the phone's/tablet's directly consume.
Unfortunately, battery apps monitor only the battery's amperage current while discharging/charging and not the phone's/tablet's directly consume when be charged.
So, the amount of amperage to which a phone/tablet limits can only correctly measured in the cable connection with ampere meter.
At least, Battery Monitor Widget would give you an idea of the amperage consuming while your navigation apps are running on the N7
Edit2:
The N7's amperage limit might be shown as specs, my 9" Sony tablet shows 5V/1.5A
Harry