Now I'm a little dubious of their calculations. An extra zero here and a zero there could throw off their predictive model by a considerable degree.
Why are you dubious?
A quick check on the interent brings up a site which declares that:
There are many different speeds of asteroids. The average speed of an asteroid is 25km/second.
25km/second equals 56,000 mph, or nearly the same speed as they used for the model.