transistors - How to control the speed of a 12V DC motor with an Arduino?

I'm trying to figure out how to control the speed of a 12V DC motor with an arduino and a 12V battery.

I want to split the «power» and «control» parts of the circuit so the Arduino and sensors receive only 5V.

So far, this is what I've tried :

I am able to control the speed of the motor by changing the PWM of pin 3 on the Arduino. This opens the NPN transistor (BUF654).

The problem is that the speed of the motor doesn't change enough.

From 0% to 50% PWM on pin 3, the motor is stalled.

Above 50%, the motor is nearly reaching its maximum speed.

I wonder if I would be able to have a linear variation :

0% - 10% : very slow
10% - 20% : slow
20% - 50% : normal speed
50% - 80% : fast
80% - 100% : RELEASE THE KRAKEN!

Here are the voltage and current the motor draws when using only a battery, or the previous circuit :

+----------------------------+---------+--------+
| Directly on 12V battery    | 12.7 V  | 61 mA  |
+----------------------------+---------+--------+
| Arduino circuit (100% PWM) | 12.47 V | 60 mA  |
+----------------------------+---------+--------+
| Directly on 9V battery     | 9 V     | 54 mA  |
+----------------------------+---------+--------+
| Arduino circuit (60% PWM)  | 9 V     | 52 mA  |

+----------------------------+---------+--------+

What have I done wrong? Could the problem come from my motor?