The current to the motor isn't the power to the motor. Current is essentially torque. The current rises quickly to a maxium value when you release the brakes. This max current is limited by wheel traction. The front motor gets less current because the weight has shifted to the rear of the car. Therefore, the front tires have less traction. Just as you release the brakes, the torque and acceleration are max but the power to the motors is low. As the car gains speed at constant torque, the power (torque x rpm) required from the motors increases. This is why the battery current is ramping up as you gain speed. Then , as you say, at around 60 mph, the combined torque and rpm reaches the power limit of the motors. As the car continues to gain speed, the rpms of the motors increase so the torque must decrease to maintain the maxium hp. As torque is current, this is why the current to the motors begins to drop off at this speed. The power to the motors and thus from the battery remains constant from this point on.