You may want to check the "rolling radius" of the tires you'll mount to the new wheels (Tire Rack may have that info online). If the difference is significant, your speedometer may be off a couple of MPH.
Curious so checked Tire Rack specs. The published rear tire revolutions per mile for the 285/35/19’s versus the Z51’s 285/30/20’s are 774 versus 777 respectively. So 65 mph with the 19 inch tire X 777/774 would read 65.25 mph. As suggested, essentially the same.
To expound a little on Mark's answer, what matters is the outer diameter of the tires, not the diameter of the rims. So, with larger rims, if you put tires on with shorter sidewall heights, then the outer diameter of the wheel (rim+tire) will remain the same.
Are you familiar with the Pxxx/yyRzz tire measurements? It is a fun little exercise to use the xxx (width of tire in mm), the yy (sidewall height as a percentage of width), and zz (inner diameter of tire = diameter of rim given in inches) to show that, for both the Z51 and base StingRays, the rims+tires are almost exactly the same outer diameter (they are oh, SO close!).
Thank you for checking that Jerry U! The Z51 rears are slightly smaller than the Base rears, so, as you found, it takes approximately three more revolutions for the Z51 to go a mile than the Base. So, with the Z51 rims+tires on the Base car, the speedometer reads a little faster than you are really going; with the Base rims+tires on a Z51 car, you are actually going a little faster than the speedometer reads. But it is all really, really close.
Also, to dougeb101, please see post #12 in the following thread for the diameter of the tires:
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