On The Autobahn!

Tom was vacationing in Germany and had rented a car. He wished to drive to Franfurt-on-the-Main and had to use the Autobahn. On this roadway there is no speed limit and he was surprised by the number of cars that zoomed past him. He devised a rough method of estimating the speeds of the cars that passed him.

He maintained a constant speed and as an overtaking car went past him he started counting the seconds until that car would reach a milestone such as a sign, a clump of trees, or some marker.(call this count "a"). Then he continued counting until he reached that same spot. (call this count "b")

If his constant speed was 120 km/hr, a = 17 s, and b = 29 s. what was his estimate of speed of the passing car? (to nearest integer)