just picked up a FRESH set of rubber for the rears today, 245/45/17 (bigger sidewall then what i wanted, but beggers cant be choosers.)

the issue is, on the road driving at ANY speed, i tested the tires out with speed and handling. and heres where the issue comes in, in turning or swerving back
and forth the car feels like the rear is on a skate board, like the *** is gonna come
under itself... very uncomfortable at 150mph!!!

my brother, who works at the tire place, said that it might be the treads are
flexing back and forth, unlike the home made drag slicks that had ZERO give.

is the reason like my brother said, or is there a sidewall strength issue..

any input would be great

PS

just figured out that they have a 99w speed rating, and my old ones had a 94y
looks like thats where the difference is maybe