Initially, the car’s speed is increasing. This causes the frictional forces to increase. With the forward driving force remaining constant, the resultant force acting on the car will be decreasing.

Since resultant force = mass x acceleration, with a decreasing resultant force, and constant mass, acceleration will be decreasing. This is represented by the decreasing gradient of the speed-time graph.

Eventually, the forward driving force will be equal to frictional forces. When this happens, resultant force acting on the car will be zero, so acceleration will be zero, and the car will be moving at constant speed. This is represented by the horizontal portion of the speed-time graph.