The main way in which Conan Doyle creates tension in this part of the story is through his vivid descriptions of what is happening.
First of all, as the car reaches the top of the hill, we know that the hill is dangerous. We are told that there are places where the grade is "one in six" -- meaning a 16% grade. This is very steep indeed.
Then, when the car starts down, the brakes fail. The narrator describes very clearly the physical and emotional feelings he has when this happens. This adds to the tension.
Then, the narrator describes very clearly the way things looked and felt as the car sped down the hill. The best example is in the following words:
The wheels were whirring like a high wind and the big body creaking and groaning with the strain.
By his use of details such as these, Conan Doyle creates tension in the story.