Wheel sizing/fitment question - 19x8.5 v 19x8 and same tire?

[netarc]

For winter running on my 2018 SQ5, I'm considering two different wheels on TR, either 19x8 +35mm / 19x8.5 +32mm. Per TireRack, the appropriate rubber would be: 255/50R19 for either wheel.

I'm surprised that both sets of wheels would use the same width (255) tires, though; I'd have thought the skinnier 8" wheel would require a less wide tire to keep same overall wheel geometry?

Unclear to me why a wheel that's smaller width would still use the same size rubber - wouldn't that change the wheel geometry/overall diameter and muck with speedo, etc?

Appreciate if anyone can explain how tire on two different width wheels would result in same geometry, what am I missing?

Thank you.

[gk1]

Wheel/Tire combo diameter is dictated by the tire dimensions.
Wheel width has little if anything to do with it since the tire contains a steel belt of a fixed diameter.

Wheel widths also accommodates a range of tire widths or one put another way a specific tire size can fit on a range of wheel widths without affecting overall diameter.
8" = usually 225 or 235
8.5" = usually 235 or 245
however these are not hard rules and Audi typically puts a 255 on the 8.5" wide wheel.
or in your case a 255/50/19 fits on a 7-9" wide wheel with 8" being ideal according to tire manufacturers.

A simple calculation tells us that:
255/50/19 has an overall diameter of 738mm on either wheel since, as mentioned, the tire's steel belt dictates the diameter.

Now as far as your other specs the wheel/tire combo on the 8.5" wide setup you mentioned will stick out from the fenders about 9mm. (Well within reason.)
Also the 255 on the 8" wide wheel will have a little more sidewall bulge than on the 8.5" wide wheel.

In general though TireRack has done their homework and they have to because they guarantee fitment. So as long as you have the right year/make/model in their configurator whatever it spits out will fit.