GPS MPH vs Dash MPH

If you don't adjust your speed accordingly, but keep the same display speed as before, then your apparent MPG will drop even more.

Not really. It will be slightly under-reading, but not by enough for anyone to sue a previous individual owner. This has been applied only to manufacturers in broad class action suits for over-reading odometers causing premature end of warranties.

Long ago, I received class action lawsuit notifications covering my Honda and first Subaru. In both cases, the lawyers prevailed over the car makers. In both cases, my cars neither displayed the alleged defects, not had any just-out-of-warranty repairs that could have been retroactively covered by the settlements. In fact, they had no relevant repairs until very long after any conceivable warranty extension.

But the lawyers did receive a very significant pile of cash to cover their (manufactured) legal fees.

 

and or would it interfere with the dead reckoning within the navigation system (steering angle and rotation).

 

I'd like to nominate this thread for being completely pointless.