1984 300SD

I was checking mileage on a trip the other day when I noticed that my odometer was not accurate. I knew I had been further than it indicated.

Upon further checking, the odometer seems to be reading exactly half of the actual mileage driven. Two mile markers go by and 1 mile rolls over on both the standard and trip odometers.

Does anyone have any ideas why this might be happening and how I can fix it? It's good for keeping the mileage low on the car, but I want an accurate record of miles driven.

Wayne