I currently own a '65 and a '66 Impala, and I'm always surprised how many people seem to mis-identify them. This seems to happen even more frequently when there is a side profile of my '65 posted online. Many people who know that the '65 has the three individual tail lights but who can't see them in the side profile, mistakenly identify the car as a 1966 Impla.
In this video - I discuss the differences between a 1965 and 1966 Impala.
Here is the rear of these two cars, and you can clearly see the difference in the tail lights. The '65 had three round tail lights on each side, which was changed out to a single bar for 1966.
The interior of these cars was different as well.
What else is different? Let us know in the comments!