From autonomy day hacker news comment: "Pegasus consumes about 500Watts, compared to under 100 Watts for Tesla's FSD computer. Elon in particular emphasized the performance per watt (as it's always possible to cram more chips to increase performance if you ignore cost and power consumption).
The comparison made in the video: 500Watts for an hour consumes about 2-3 miles of range. In a city in slow traffic, going 12mph, that's a significant range reduction. So you might have a 10% improvement in range for the Tesla ASIC in low speed conditions" [0].
Also, Tesla is/was planning on running these chips even while not in autonomous mode in order to spot new scenarios which it can then record what the car sees and what the human does in order to collect a lot more unique road scenarios to train their models with.
[0] Credit Robotbeat: https://news.ycombinator.com/item?id=19729743
