Performance of RC car under different gear ratio, measured data vs hypothesis

Assuming the driveshaft of the rear wheeled drive RC car follows the linear DC motor theory for stall torque and no-load RPM, also voltage supplied is constant and no inductance as well. Note: the motor is running above its operating voltage 3V, the motor voltage supplied was measured 5V.

Motor used: product.mabuchi-motor.com/detail.html?id=9

Forming hypothesized result: speed are directly proportional to voltage, I directly multiply the voltage ratio with the expected speed then multiply again by the gear ratio. Gear ratio was verified by calculating its' teeth. Actual applied voltage was measured using a multimeter and measure the voltage across the motor. The hypothesized stall torque were also multiplied by the same voltage ratio and the gear ratio.

The base of this experiment uses the formula

ω=Vk-Tk2*R

where the ω is the no-load speed, v is the voltage, k is the electrical constant or the torque constant assuming they are equal to each other.

Hypothesized results of a 7.3 gear ratio
Stall torque=0.065