[Lee Dalton]

Pretty much as Paul said. Miles per kWh of electric, compared to miles per gallon.
Electric at whatever price per unit, and fuel at current petrol/diesel prices.

To get to the miles per kWh I think you need to know the starting point, ie. the max usable battery capacity, then work backwards using the state of charge. Presumably you can pull mileage out of the CAN messages ?

 

[Lee Dalton]

Can you please elaborate on how to calculate miles per kWh?

You have to take a point in a journey, and work out the number of miles covered since the start if the journey, divided by the kWh of power used.
I think if you try to calculate it continuously, you will get a wildly varying number, much like the instantaneous read out on a petrol car.