Is there any problems when you downsize a tire and a rim from what the manifacturer tires and rims are on the cobalt ss/sc?? If so which problems do u get??

http://www.cobaltss.net/forums//showthread.php?t=8149

has a wheel size calculator maybe this will help

The most important thing is whether or not you stay with the same size diameter overall of wheel + tire. For example, on the SS/SC if you wanted to go with a 17" rim, but wanted the diameter to stay the same (no difference in speedometer or odometer), you'd have to go with 215/50-R17 tires.

If you change tire AND wheel size which results in an overall difference in diameter, then your speedometer and odometer will be off until you get the computer reflashed so that the car adjusts for the new diameter. With no adjustment, a larger diameter will cause the odometer to advance slower and the speedometer to read a slower speed than you are actually going. A smaller diameter will have the opposite effect.

Most performance shops will do a reflash for you for about $50-100 which will adjust for any tire size you decide to go with.

Hope that was helpful...