After ending the year on a good note, Toyota execs are pledging that their Prius brand or hybrid vehicles will be the best selling cars in America by 2020. Prius sales were at its best-ever in December this year, moving over 15,000 units last month and over 140,000 total units for 2010, or 8% of Toyota's total US sales. That's a 0.9% increase over 2009.

The Japanese auto maker is…