My BMW i3 -- How Good Is the Mileage?

The i3 is rated by the EPA at 117 mpge, which is their way of looking at the cost of electricity and the cost of gasoline, how far a car goes per unit of energy consumed and making a rough comparison with the actual mpg for conventional cars.  Obviously EPA has a very complicated process to calculate mpge and has to look at types of driving conditions and the costs of gasoline and electricity across the country.

Here's my simple way to calculate mpge:
 
I also have a Toyota Prius (2nd generation) that averages about 45 mpg, and at $3.50 per gallon, this would result in about 13 miles per dollar.  The BMW i3 can go about 75 miles on a charge that costs about $2.00 from APS in Arizona, and this would be about 38 miles per dollar.  So the BMW i3 is about three times better than the Prius.  If I multiply the 45 mpg by three, I would arrive at 145 mpge for the i3.  This would obviously change if the cost per kWh from an electric utility company is different and the cost per gallon of gasoline is better or worse than $3.50.  If your conventional car gets 22.5 mpg, the i3 would be six times better.

My i3 also has the range extender, a two-cylinder BMW motorcycle gasoline motor that is activated when the charge on the battery drops to 6% of its capacity and adds about 70 miles to the i3 range.  It provides 35 hp to the car, so when the battery is low, the car runs on 35 hp rather than 170 hp.  This obviously limits the performance of the i3, especially on hills.  My BMW advisor drove the car from Phoenix to Prescott, a trip of about 100 miles with an elevation rise of about 4500 feet.  He had to resort to the range extender for the last part of the journey and was limited to about 45-50 mph.  The gas tank for the range extender is 1.9 gallons of premium fuel and the EPA fuel economy for it is 39 mpg.  The small size of the tank results from California's strict rules for classifying the BMW i3 as a zero-emission vehicle.  Since I plan to use the i3 in the Prescott area, I do not intend to use the range extender unless in an emergency.