tire size calculation error?

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) in hptuners. 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 i'm missing here?