I think alot of people running different than stock size tires on stock rims should know what kind of effect its going to have on your speedometer. With Hp Tuners its a simple fix, but not everyone has HPTuners. I wouldnt go with a different tires size then what is listed here, because your going to be screwing with the actual mileage on your car. If you speedo is reading 65MPH, but your only going 63.56MPH then over time your car is going to report more miles then it really has. Likewise if your speedo reports 65 MPH and your really going 66.06 your car is going to have less miles than what the odometer reports. Over say 50,000 miles this can add up to a few hundred if not thousand extra or less miles.

I only posted what i thought were the best 3 tires sizes for each rim, going any other size is going to have a significant effet on the odometer. If you would like to check out some other sizes, you can use this http://www.discounttire.com/dtcs/infoTireMath.dos calculator.

Cobalt SS Supercharged

Stock Wheel Size : 215/45-18

225/45-18 Speedo=65 Actual Speed=65.89
225/45-18 Speedo=100 Actual Speed=101.37
235/45-18 Speedo=65 Actual Speed=66.80
235/45-18 Speedo=100 Actual Speed=102.77
235/40-18 Speedo=65 Actual Speed=64.44
235/40-18 Speedo=100 Actual Speed=99.14

Cobalt SS 2.4L

Stock Wheel Size : 205/55-17

215/55-17 Speedo=65 Actual Speed=66.08
215/55-17 Speedo=100 Actual Speed=101.66
225/50-17 Speedo=65 Actual Speed=64.95
225/50-17 Speedo=100 Actual Speed=99.92
235/50-17 Speedo=65 Actual Speed=64.85
235/50-17 Speedo=100 Actual Speed=99.77

Cobalt 2.2L

Stock Wheel Size 205/55-16

215/55-16 Speedo=65 Actual Speed=66.12
215/55-16 Speedo=100 Actual Speed=101.73
225/50-16 Speedo=65 Actual Speed=64.95
225/50-16 Speedo=100 Actual Speed=99.92
235/50-16 Speedo=65 Actual Speed=65.97
235/50-16 Speedo=100 Actual Speed=101.49

 

or you could just go here, punch in your ****, get the % error, and do some math... *shrug*

 

Helpful post. That will be good for those who just want a quick reference. Thanks alot for sharing.