| I was running a 275/40R17 on my 17x9 wheels, and a 275/35R18 felt a lot crisper on my 18x9.5s. Granted there's going to be a difference with the shorter sidewall, but also the sidewall isn't pulled in on the 9.5" wheel.  vs 
Notice how the tread is pretty much wider than the wheel on the 9" wheel. This makes kind of an inverted trapezoid that is naturally unstable. This is part of the reason that a lot of German cars come with the tire slightly stretched on the wheel. Below is a sketch of an ideal 275 tire on a 9, 9.5 and 11.5 wheel. Geometries definitely change as you go from ideal into the real world, but these illustrate what happens as the tire deforms in cornering. When the tread is wider than the wheel, it makes it easier for the tread to roll over the wheel. When the wheel is wider than the tread, the outer sidewall gets closer to making a straight line with the tread, limiting its displacement. 
Granted, a car's sensitivity to this tire roll will depend on setup. Stock tension rods have a lot of displacement on turn in as well, so the effect of a tighter sidewall will be less noticeable than on a car with solid joints. What will be noticeable across both cars is traction and tire wear. As the tread rolls over, you end up only riding on the shoulder of the tire instead of the whole surface. This the whole tire from working for you and prematurely wearing the shoulder of the tire. This can some times be mis-diagnosed as under-inflation. FWIW, I'll probably be stepping down to a 255/35R18 for my next tire. For one that's as wide as the RE-71R comes on an 18, but I could also benefit from the slight stretch on the sidewall.
-Dan
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275 is a little wide for a 9 - 
