Why would a Audi A8 shake at 40-60 mph?

Different wheels in front and rear. example: 245/45/17 front wheels and 225/50/17 rear.

cause of this is the different spinning of the front and rear wheels. as u know all quattros have to get same tires on all rims. maybe i helped someone. (had this on my A8 shake hard at 60km/h)u can seriously damage your car driving like that. check your tires MUST be the same size.