[Wilt]

I just thought I would share my experience, so that others considering the same action would have heard real world experience...
I recently purchased an inexpensive aftermarket add-on HUD unit for only $15 from Temu, in the hopes it would connect and work, but knowing only $15 was at risk if it didn't work properly in my 2021 228i xDrive Gran Coupe.
Happily, it works quite fine for my needs (speed disply only), as my car did not come with the factory HUD option. While it can show engine temperature or a few other things at the press of a button, I simply want speed display. I grew spoiled having the HUD in my wife's new Toyota and driving her car during a road trip in January.
Installation is super easy...plug the cord into the OBD connector under the dash, press some buttons to default it to MPH display rather than metric, stick it on the dash (no reflective sticker is needed in the 228i Gran Coupe, the windshield itself is fine!).
I placed mine forward on the dash closest to where the windshield meets the body, to position the digital reflection down low where it is not so distracting. It is readily seen during the day, and it dims itself after dark.
Interestingly, the digital value displayed on the HUD does not match the BMW dash digital value... it is 1 mph slower than the digital display on the dash, , which is great to see as the HUD display speed is closer to reality than the designed-optomistic (by German regulatory law) speed display on German automobiles. At highway speeds the HUD displayed speed is still 3mph optimistic compared to the real overground speed of GPS...that beats 4mph optimistic!

[Wilt]

The HUD shows an indicated speed which is 1mph slower than the digital dash display, and both are optimistic numbers in conformance with German regulatory requirements. For example, digital dash = 70mph, HUD add-on = 69mph, real speed over the ground (Garmin GPS) = 66mph.
The optimism is not so great at street speeds, so the offset is a percentage rather than a fixed margin.