On my 550i, the speedo reads 2 mph high on the highway. at 80 mph on the speedo, the gps on my phone holds a steady 78 mph. I have the 19" Sport package wheels.

 

I think the factory sets the speedometer faster than actuall speed for liability purposes

 

the overall circumference of a 245/40/18 and the 275/30/19 or 245/35/19 are pretty much the same, it's not going to affect your speedo readout...i would think your little 3rd party device that relies on whatever technology (gps etc) is off. Besides does 1-4 mph really matter?
I had the same 123's stock and have had the same 19" tires as well...there isn't a difference, and if so so slight that I could care less

 

The difference is of course not that important or noticeable. It however is always good to know. Like you guys said the larger the tire/rim combo the slower it should go. That's the reason I asked this to see if there was another underlying cause. Which seems like its common to have stock/OEM options on the 5'ers ranging from 17" to 19"...

 

I think the general "rule of thumb" I have always heard still rings true: If the overall rolling diameter of the replacement wheel/tire combo is within 3% of stock there should be no noticeable difference or need to adjust the speedo.