As woodj says, there are no issues with clearance for the 285/40 tires on 9 or 10" wheels. The only issue may be if you are getting the full performance potential of your tires when you mount the 285s on a 9" wheel. The sidewall angle would be better on a 10" (or even wider) wheel. Better sidewall angle allows the tire to perform better in cornering/lateral traction. The Tire Rack web site has a reference which can be used to match the best wheel width for each tire width.
'06 GT, manual, Satin Silver, lt. grey cloth int., IUP.
MODIFICATIONS - Goodyear F1 GS-D3s; 245/45 on 18x9, 275/40 on 18x10| Progress Technology springs & sway bars| Steeda adj. panhard bar, panhard brace, front control arm brace & bushings| strut tower brace| Tokico D-Spec dampers| CHE adj. LCAs & BMR relocation brackets |drilled & slotted front rotors| C&L intake| BamaChips SCT tune| FRPP 4.10 ring & pinion| Pypes off road H-pipes| FRPP GTA mufflers.