for a small, aerodynamic car, the two biggest sources of drag on a car are rolling resistance and aerodynamic drag. Rolling resisitance is more or less a constant value that is proportional to the weight of the vehicle. Commonly it is around 1 to 2% of the weight of the vehicle. The aerodynamic drag is a product of the Cd A (drag coefficient multiplied by frontal surface area) of the vehicle and the square of your speed. So at lower speeds, rolling resistance is the biggest factor. At higher speeds, aerodynamic drag is the largest factor. The "crossover" speed is usually around 40mph. There are also losses in the motor, drivetrain, and electronics.
Your mileage may vary, but for all my calculations I used a combined efficiency of 78% for battery-to-wheels energy conversion efficiency.
Look here for the CdA of mazda vehicles:
http://www.mayfco.com/mazda.htm
this page lists an '86 RX7 with a CdA of 0.55m^2 which is a good number.
http://en.wikipedia.org/wiki/Drag_(physics)
http://en.wikipedia.org/wiki/Drag_coefficient
Rolling resistance is affected by your tires, brakes, alignment and a host of other factors, but if you know the weight of your vehicle and the CdA, and you have about a mile of flat, deserted roadway available you can measure it using a coast down test and some math.
