diff --git a/src/ground_vehicle.hpp b/src/ground_vehicle.hpp
--- a/src/ground_vehicle.hpp
+++ b/src/ground_vehicle.hpp
@@ -377,6 +377,41 @@ struct GroundVehicle : public Specialize
 			this->gcache.last_speed = this->cur_speed;
 		}
 	}
+
+protected:
+	/**
+	 * Update the speed of the vehicle.
+	 *
+	 * It updates the cur_speed and subspeed variables depending on the state
+	 * of the vehicle; in this case the current acceleration, minimum and
+	 * maximum speeds of the vehicle. It returns the distance that that the
+	 * vehicle can drive this tick. #Vehicle::GetAdvanceDistance() determines
+	 * the distance to drive before moving a step on the map.
+	 * @param accel     The acceleration we would like to give this vehicle.
+	 * @param min_speed The minimum speed here, in vehicle specific units.
+	 * @param max_speed The maximum speed here, in vehicle specific units.
+	 * @return Distance to drive.
+	 */
+	FORCEINLINE uint DoUpdateSpeed(uint accel, int min_speed, int max_speed)
+	{
+		uint spd = this->subspeed + accel;
+		this->subspeed = (byte)spd;
+
+		/* When we are going faster than the maximum speed, reduce the speed
+		 * somewhat gradually. But never lower than the maximum speed. */
+		int tempmax = max_speed;
+		if (this->cur_speed > max_speed) {
+			tempmax = max(this->cur_speed - (this->cur_speed / 10) - 1, max_speed);
+		}
+
+		this->cur_speed = spd = Clamp(this->cur_speed + ((int)spd >> 8), min_speed, tempmax);
+
+		int scaled_spd = this->GetAdvanceSpeed(spd);
+
+		scaled_spd += this->progress;
+		this->progress = 0; // set later in *Handler or *Controller
+		return scaled_spd;
+	}
 };
 
 #endif /* GROUND_VEHICLE_HPP */