Empty in the summer anywhere from 17-21, towing 8-12. Empty in winter (winter grade gas) 16-19. Hand calculated? No I read what's on the mileage display.
Looking for mileage while towing a camper, mine is 5000 lbs. I know what my 5.3 gets (not good) curious to know what others are getting with their GMC Duromax diesels