Every 60s mechanical speedo, and many from a whole lot later, when run in reverse will decrease the odometer mileage. I have 5 cars through '89 and they all do that. Its a direct gear drive; drive it backwards and it counts backwards. The needle would also display negative MPH if it weren't for a little peg inside that prevents the needle from going below zero. Used car crooks used to spend a lot of time with a reversible drill running the mileage backwards before selling a car. If they were smart, its easier and quicker to take the speedo apart and set the odometer manually to any mileage you want. Congress in the 70s spent a lot of time trying to figure out how to keep people from turning back odometers and is the main reason for the 'I have no idea what the mileage is' statements you now have to sign to sell a car.