From 8: 23 to 8: 27, the speed was constant, so he drove at a constant speed during this time, so B was correct.

As can be seen from the figure, during the period from 8: 20 to 8: 30, the highest speed of his car is 60 km/h and the lowest speed is 40 km/h, so the average speed must be less than 60 km/h, therefore, C is correct and D is wrong.

So choose D.