Lots of threads about donut spare. "How fast, How long to drive, etc" MY POST IS ABOUT THE DONUT SIZE. .

Sold my A8 a couple weeks ago. until a replacement, I'm driving wife's grocery getter, a 1999 A6. When bought new original buyer could have 195/65/R15 or optionally, 205/55R16. Diameters of standard or option are 24.9" and 25" respectively. I bought it from PO 6 months ago with Continental Control Contact SRS 235/45-R17 which have a diameter of 25.3" No big deal since all four are the same.

Apparently, Audi came with a donut T125/90/15 Donut diameter is only 24.9" WTF? That's a full inch smaller than either original or option tire.

The left front got soft and I tried to make it to a gas station for air. I didn't make it. Put on donut on after a half mile. So yeah, I didn't drive over 50 MPH to 40 miles home. Next day, at shop, I realized it was dumb to drive that 1/2 mile flat. Tire is ruined. No one locally or online has the Continental Control Track SRS. I can't even find it online (lots of other Continental "contact" models but no "Control Contact"

I found a used exact tire online with 9/32nds and "no repairs". Current tires also have 9/32. It will be here in two days. With no other car, I had to drive 6 miles to work for the last three days and for two more.

"MISS MATCHED LEFT-RIGHT TIRES CAN DAMAGE QUATTRO" I understand even uneven wear differences can cause a problem

An inch difference is a hellofalot more than a few 32nds wear.

So I already ruined the tire, Am I killing the Quattro differential? How would I know? Does it make a noise, etc?