If the truck travels 10 mph forward, not on the treadmill, and the treadmill goes in reverse 10 mph, wouldn't that make the plane travel more than the 10 mph you posed in your initial question?? Because you are now adding the pulling away of the truck as well. Because the treadmill, although spinning 10 mph, cannot move itself 10 mph forward as well, only the belt...so the tow truck itself is moving away from the treadmill. What if both the treadmill and the plane are pulled forward by tow trucks?? Thus making it equal again as you stated in your initial question.