On the other hand, Mallard, with its famous 126mph run, and largeish 80" drivers, was doing 5280 x 12 x 126 inches, divided by 80 x pi is 31513 revolutions per hour... 525 revolutions per minute, 8.7 per second. Your rule-of-thumb seems reasonable. 8 revs per second is hammering along, 5...