You need a good Pulse Width Modulator between the motor and the battery. This will, at bare minimum, cut your power usage in half and keep the same power to the wheel.
The pwm is a bank of parallel power transistors (2n3055's are good, 6 for 300w) and a pulse controller for that bank. You need a pulse-control circuit that has independant frequency and duty-cycle settings.
You need about 20amps worth of 400v diodes across the motor connections REVERSE BIASED. This keeps the emf inside the motor and not out trashing your transistors.
The battery positive goes to the motor. The motor negative goes to the common collectors on the power transistors. The emitters go to battery negative.
Put your scope across the motor connections so that you can see the voltage that will appear across the motor.
Your scope will show you what's happening. Turn the pwm frequency down. Then send a slow train of single pulses to the motor.
Adjustment #1: A properly tuned pulse-width
You should see a downward "ramp". Adjust the pulse-width for the maximum ramp length without allowing it to become a DC flat-line. This means the pulse is cutting off JUST BEFORE the motor begins to take steady DC from the battery. This is the absolute maximum pulse-width that should be used.
Adjustment #2: Maximum pulse-frequency 50% duty
Now turn the pulse frequency up until you have very close to a 50:50 on:off ratio. Let the OFF time be slightly longer than the ON time. LOCK the FREQUENCY of the PWM right there.
The throttle is now wired so that it varies the PULSE-WIDTH between ZERO and MAXIMUM on the PWM.
No matter what the pulse-width, the locked frequency means that the motor windings are always operating at very near their own internal resonant frequency, maximizing efficiency.
