If your speedometer was calibrated for the 21" (with a 95.4" circumference) and you were driving 40mph, you would actually be going 41.8mph on the 20" (with a 100.2" circumference).
I don't know if Waze can calculate it that close.

 

My experience on various vehicles have been that the manufacturers deliberately have the speedo over read by a few % (2-5% in my experience), to avoid drivers going faster than they think as there are differences in the diameter/circumference of different tire manufacturers even with the same spec. In the D5 case most of the stock tires are close to 31" diameter. I looked up the Pirelli specs for the difference OEM offerings on the D5.
19" wheel - 255/60/19 - 31.1 " diameter
20" wheel - 255/55/20 - 31 " diameter
21" wheel- 275/45/21 - 30.8 " diameter
22" wheel - 285/40/22 - 31 " diameter

Since circumference is just diameter x π we can just use the diameter to calculate the difference in speed for the same rotational rate.

Not sure which tires you have in 275/55/20, but the Continental ATs (which I'm looking into) is 31.9" diameter i.e. 31.9/31 ~ 2.9% bigger or 31/31.9 ~ 2.83% that it will lower the speedo reading. So it is no surprise that your speedo is probably very close to spot on now. Waze is not the most accurate as the phone GPS is so-so for this use, but you can probably get a better GPS to get an indication.

Also keep in mind that it will lower your odo reading by the same ~ 2.8%, which will impact your MPG just from an instrumentation point of view, never mind the extra weight and rolling resistance. Have you noticed any difference in MPG?

Will be interested to hear if you have any rubbing on or offroad, especially if you don't adjust the suspension.

 

I test fit the 275/55R20 on my D5 and they clear just fine as far as I can tell. Their diameter is 31.9 vs 30 for the stock 255/55R20's. So with 0.9 larger diameter they only take up about 0.45 inches of the clearance - half the diameter.

 

 

This site will give you a good idea of what your new speed will be compared to your old speed based on tire size:

https://tiresize.com/speedometer-calibration/