That's why I use the Excel file I've put together whenever I'm contemplating changing tire/wheel combinations. The MPG multiplier in my file will give you the Speedo adjustment compared to original OEM setup. Pretty simple really. Larger diameter tire/wheel combinations will result in faster actual speeds vs. the indicated Speedo value. Smaller diameter tire/wheel combinations result in slower actual speeds vs. indicated Speedo value. Everything is linear, too, across the entire Speedo range.

So, I agree with SRT SIX comments above. The 285/30x19 tire, if it fits the OEM rim & suspension w/o alterations required, probably gives the best fitment since it's slightly smaller in overall diameter. You probably give up a little on ride quality due to smaller aspect ratio. (Yeah, like we bought our XFires for it's soft & supple ride! It's a 2-seater!!) If you drive aggressively, you'd always be going a little slower than your Speedo indicated (but not much!).

However, as Gary has stated in his thread on the Hankook Ventus tire, the 285/35x19 also fits the stock rim & suspension without the need for spacers or alterations. And the overall larger diameter gives the XFire a slightly more "wedge-like" profile since it's raised up about 1/2" in the rear. Is that enough to affect headlight aim and other aspects related to vehicle height? Probably not enough to notice. But remember, for a Speedo indicated 75 MPH, you'd actually be doing 77.4 MPH! And if you're like me and you "ride that edge" of +10 MPH that most Law Enforcement officials will give you on major highways, in a 65 MPH posted speed limit, I'm now doing 77+ and just got busted!! (Especially on the stretch of I-385 that runs through Mauldin!)

In either case, the extra 20-30 mm of tread width really fills the rear wheel wells and gives a great looking stance from behind!!! Probably will purchase wider rears when I need to replace OEM Conti's.