As someone stated, the actual odometer would be required to record all mileage, regardless of source.
There could be all kinds of information we could get from the vehicle. Miles on electric would be useful, as well as charge cycles - both of which could be used for calculating longevity of the battery system.
For the generator, we'd need to realize that while it is a four cylinder engine similar to regular cars, but it is a generator. It's hours of operation should be logged. Distance traveled and total power generated could be useful.
I would bet a lot of this data is already designed to be logged by the system. Whether or not we will have access to it through the vehicle's interface remains to be seen.
"The balance of power is the scale of peace. The same balance would be preserved were all the world destitute of arms, for all would be alike; but since some will not, others dare not lay them aside. Horrid mischief would ensue were one half the world deprived of the use of them; the weak would become a prey to the strong." -Thomas Paine