BMW Factory Wheel/Tire 19" vs 20"
Diameter 29" vs 28.7"
Sidewall 5.0" vs 4.3"
Circumference 91.2" vs 90.1"
Revs Per Mile 716 vs 724
So, the actual height difference is just three tenths of an inch (using the difference in diameter)
However, IMO, the perceived height difference could be more than this. One reason why I think the "perceived" height could be more on the 19" is the particular 19" tire could have been a little tippier than the 20". The sidewall of the 19" is 5.0" vs 4.3" - although this is only .7" it is .7" below the rim and .7" above the rim for a 1.4" difference in sidewall height, top to bottom. And, this is a 16% difference in sidewall height, which I believe is easily felt in the handling department.
I have the 19s, and although I love my 5.0 Sport AD, I do not like the wheel/tire setup - I don't think it is anywhere ideal (yet). I plan on "firming" up my car's handling with a) different tires
on the 19s beginning with non run flat UHP Vredestein winter tires
and b) purchasing 20s this spring.
In this case what is perceived
is for all practical purposes, reality