Answer: The narrator is not entirely truthful.
In this excerpt, the narrator is describing the reasons why he has decided to stay in Venice longer. He explains how he persuaded his friends, and almost persuaded himself, that the reason for his long stay was the deisre to study Venetian culture. However, he implies that there are hidden motives for it, indicating that the author is not being entirely truthful.
This "italicized" part of the sentence is an adjectival dependent clause. It acts as an adjective to the word motorist and cannot stand alone without putting it with an independent sentence, such as "The motorist lost his way."
The word you could possibly be looking for is "intent"
