mismatch between speedo and odometer

I'm not sure if this is where this belongs, but here goes. I built a 70 Dart with a 318 and an A500 4 speed overdrive automatic (RH42). The speedometer was the best of 4 that I had as tested for me by someone who does dash gauge restoration. The speedo works fine - steady without fluctuation and the odometer clicks off the tenths and the whole unit seems to work fine. I have a GPS in my car that shows current MPH. It seems to be very accurate when you are going straight and steady, so I used it to figure out calibration of the speedometer and for once it seems to be dead on with the speedo gear now installed. I'm on a looong trip and have had some opportunities to check my odometer against 3 interstate measured 10 mile marked sections. In all cases the odometer records 9.0 miles for every 10 miles traveled. Has anyone ever experienced this??? It's easy enough to do the correction for mileage and distance traveled, but this is just weird... Any Ideas????