At the college I go to I can look into the automotive database that the dealers have. It says that when upgrading the wheels on the cobalts you have to have the speedometer updated and the something about the PCM. It says that the PCM must be updated to the wheel size to determine the stiffness of the steering of the. Do you really have to updated the PCM with the dealer?

 

Most performance shops can do it for you as well, but yes, if the overall diameter of wheel + tire changes, you do have to have the PCM recalibrated.

 

but does just have to do with the speedometer?

 

No, your odometer will be off (and also fuel mileage/economy since it's inaccurately recording miles traveled).

 

so if i have a bigger rim will it be putting more miles or less miles on my?

 

Depends on what size tires you put on. Rim size itself isn't what they're talking about, what they're talking about is overall diameter of rim + tire. Like now, for example I have 205-60/R16 snow tires on my SS. Stock size is 215-45/R18. Overall, my tires are .1" BIGGER than stock. So I'm logging slightly lower mileage, but my speedometer is slower than it should be (60mph on the speedo is really 60.2mph). For anything more than a couple tenths of an inch you really need to get the computer reflashed.

Here is a good tool to use, it'll tell you what the differences are between the two or you can use the bottom one to see what tire size you need to stay closest to stock.