In short, now that he knows (or, anyway, prefers to believe) that the speedometer has been understating his speed, and that the shimmy therefore doesn't start until he is really going 55, he thinks that he can drive 55 as measured using that same speedometer . Uh-uh.
The speedometer has been understating his speed but he now believes he can measure 55 using the same speedometer as it starts when he is going 55.