I've had a number of readers ask me what the typical tractor-trailer combination truck weighs. The easy answer is "there is no easy answer" because the variability is huge. Luckily I recently found data from Oak Ridge National Laboratory on a 2008 survey of 11,365 trucks in 15 states. ORNL graphed the weight distribution as follows.
The EPA estimates the typical weight of an unladen Class 8 tractor-trailer combination is on the order of 35,000 pounds while payloads typically max out at about 40,000 pounds. So while 80,000 pounds is the load limit for Class 8 trucks, well over 90% of the tractor-trailers weigh in at less than 73,000 pounds.