Originally Posted by crammer

Anytime you're over an agricultural area of the midwest (and seated in a window seat), where much of the plains are partitioned into squares one mile in width, you can easily calculate your ground speed using a watch. It helps to have a fixed point of reference on the plane (the wing works well), but simply time how many seconds it takes to traverse a bunch of squares, and divide the number of squares by the time, and you will get your speed in miles per second. Then multiply the result by 3600 to get mph.

For example, if 5 squares takes 45 sec, then this is equivalent to a ground speed of 400 mph: (5 miles / 45 sec) * 3600 sec/hr = 400 miles per hour.

(Much easier of course is to fly hub to hub in a 3-cabin 767 or 777 and view the airshow!)