I've sold Audis for about 2 years now at a dealership and also a proud owner of a white 2016 Audi A3 e-tron. From my experience, there seems to be an average of 2MPG drop with every inch of wheel diameter increase. 16in wheels I've seen avg MPG of around 39-40 (without charging, only hybrid mode). My e-tron came with 17in wheels, I get an average of 36-37MPG (without charging, only hybrid mode). One of my co-worker, he installed a set of 19s from a S3 and clocked around 33-34MPG avg only in hybrid mode. Since 16in wheels get the best MPG, that's why Audi's e-tron ULTRA only comes with 16in wheels.
This is really helpful information.
Do you guys think the spoke design has any effect on mpg efficiency? I have no idea of how the physics of a spinning wheel's spoke's affect wind resistance, but it's interesting to think about.