Going from 205/50/17 to 235/40/18 is a big jump, from what ive been able to calculate there is a 1.3% difference in the tire size causing the speedo to read too slow. So I went into hptuners to recalculate the tire size for the speedo when I noticed something unusual. Stock tire size for the SS/NA is showing as 24.29 (or 24.3) and for the LSJ its showing 25.63 (225/45/18). If im not mistaken both of these numbers are incorrect to begin with. Following heights should be...

205/50/17 = 25.07
225/45/18 = 25.97
235/40/18 = 25.40

Am i incorrect in making the change from 24.29 to 25.40 in hptuners, is there something else im missing here?

 

Is the car is programmed for a 24.3" diameter (which would be correct for a base model Cobalt with 195/60R15) ? Regardless of the starting point, you'll need to change to 25.4". We haven't had much luck with 235/40R18 fitting without some fender mods etc, but I'll assume you checked all that out already.

 

correct. ss/sc runs a 215 45 18 tire. ss/tc runs a 225 40 18.