TL;DR: Switching from OEM 19" to 21" wheel & tire set means you will actually be traveling at 65.17mph when your speedometer shows 65mph, and will be actually covering 100.3 miles for every 100 miles logged on your odometer.
Josh, the diameter of the tires on the 21" wheels vs the 19" wheels are virtually identical. As such, switching from one to another should have virtually no effect on speed or milage display.
For anyone interested in more detail:
The tire size for the 21" rims is 245/35-21 and the tire size for the 19" rims is 245/45-19.
245 indicates the width of the tire track in millimeters.
The '35' and '45' numbers refer to the sidewall aspect ratio. '35' means that the height of the tire sidewall is 35% of the width of the tire track. For the 21" wheel the tire sidewall height is (245mm x 0.35) = 85.75mm. For the 19" wheel the tire sidewall height is (245mm x 0.45) = 110.25mm.
To compute the diameter of the tire, we add the diameter of the wheel + 2x the height of the sidewall. This means that the diameter of the 21" tire is (21" x (25.4mm/1") + 2(85.75mm)) = 533.4mm + 171.5mm = 704.9mm. The diameter of the 19" tire is (19" x (25.4mm/1") + 2(110.25mm)) = (482.6mm + 220.5mm) = 703.1mm. This calculation yields a diameter difference of 1.8mm between the two tires (tire for 21" wheel is 1.8mm larger).
To compute the actual speed vs displayed speed when switching between tires of different size, divide the diameter of new tire by the diameter of old tire (this is your conversion factor), then multiply by the target speed. So if your speedometer is calibrated for the 19" wheel & tire set at 65mph, switching to the 21" wheel & tire set means you are actually traveling at (65mph(704.9mm/703.1mm)) = (65mph * 1.003) = 65.17mph when your speedometer reads 65mph.
To compute the actual distance traveled vs displayed milage, multiply your conversion factor by target milage. So for every 100 miles you log on your odometer on a car calibrated for 19" wheels that is actually equipped with 21" wheels, you've actually traveled (100mi * 1.003) = 100.3 mi.