Two taxi stories:

In London, all bleary-eyed after a long trans-Atlantic flight, I mistakenly gave the driver a very large tip for the ride from Heathrow. He chuckled and gave back more than half of it saying what he was taking was more than sufficient.

In New York, on a rainy cold night, I purposely rounded up the fare to include approximately a 25% tip. The driver reached back, took the money, counted it, and said neither "thank you" or "good night."