the problem with scales is some set-ups are not repeatable.

examples:

NASA event at Putnam: weighed the car before qualifying. qualified and raced and re-weighed the car. gained 10 pounds. better than infinite mileage since i was making gasoline.

NASA Nationals: weighed the car. ate a great meal at Sam Myer's paddock area and bought breakfast from his family's stand/trailer the next morning. lowered left rear spring perch for changing corner weights and gained 8 pounds. now i can chow it away but no way did i eat 8 pounds of food!

bathroom scale for a 5 gallon can and weigh before and after re-filling the car could work.