What axle in the eco diesel? Is it a rear diff mounted speed sensor like just about all the other dodge trucks?

If so, a gear ratio change won't effect speedo calibration because the tone ring doesn't care about gear ratio. It rotates with the ring gear and the tires, so it makes the same number of revolutions per mile.

That said; changing tire size WILL alter speedo calibration because now the tone ring for the speed sensor on the ring gear no longer has the same rev/mile.

This can be reprogrammed with a dealership diagnostic scan tool. Not your generic part store code reader. I have a snap on MT2500 that I recalibrated the soeedo in my Dakota with.

The snap on versus, solus, and modus scanners will work too when they have software loaded that's current for your model year truck. FYI the speedo calibration is done through the anti-lock brake controller, not the main engine computer, which is why a generic emissions code reader don't work. (It's still connected through the obd-2 port under the dash though)