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.

 

Kane and I are on the same page. While a 285 may look and sound good, I wasn't willing to chance it. A lot of guys here have and had no issues. Too each his own. I have no problem there. Just sharing my opinion. ...

With the front, without spacers the 235/40 is VERY close to the nut. I posted pics (check my gallery) when I installed the 235/40/18 up front without spacers.
It made me nervous so I didn't drive the car until my front spacers arrived. I went 10mm up front and 12mm in the rear with 275/35/19 on stock rims. It turned out really nice. The tires now fill the wheel wells without sticking out too much or being sunk in like the factory settings were.

Good luck and enjoy!

Cheers,
KrzyFast

 

I actually run a 235 35 front and love the feel!! and your totally right on the puny 225>lol...they just don't look right inside our front wheel well...Im lowered on the eibach pro kits and I think it looks just great with 235


And Kane I would totally agree on the 285's being very border line on the stock wheel, you can get a away with it, but on the track I just think that beed would unseat....

 

This what I'll be putting on soon.

I've ordered new tire and rims. The Hankook Venus V12 evo K110 with 235x40X18 paired with 18X8 Touren TR6 Rims on the front and 285x30x19 paired with 19x9.5 Touren TR6 Rims on the back.

 

Just in case everyone missed the little gem that is in th documentation library in the SRT6 section... I put a wheel and tire spreadsheet up that has auto calculations for overall tire height, width difference vs. stock, offset calcs. and much more.

Check it out in the SRT6 section (sticky).

 

I'm running a 285/30 on the OEM 19" wheel with no issues...

 

My wheels and tires are in and ready to be thrown on. Still waiting for my brakes that I ordered from a member on the forum. Unfortunately they were shipped ground and well, still waiting. Hopefully they come in soon, and I can post some pictures of the new wheels with the 285-30 on the rears. The tires look great right now. Super wide!

 

Need pictures!!!!

 

A 285/30/19 on OEM wheel:

 

WOW! A white SRT.... too cool!

John, that is a great looking car.... if only it were true

 

Look closely.... IMO it looks like an NA with a grafted on SRT6 rear spoiler.

 

I think Mike caught that LOL...

 

Thanks buddy! Check it out in about a week... there's another substantial upgrade coming!

 

Something I noticed when I was searching for tires was that the tread widths vary from differant manufacturers for the same tire sizes. I seem to remember that the Conti DWS's tread width was slightly wider than other tires of the same size. Something to look closer at if the 285-35s are being considered, although the width between the beads should be standard. This may cause some rubbing if mounted on a lowered car and require spacers to correct. I went with 275-35s to play it safe.

One other thing I ran into is that a couple of tire shops refused to mount tires that were 3 sizes over OEM. Something about liabilty blah, blah, blah. These were usually the franchise stores, but I found a Mom & Pop place that didn't know the difference.

 

295 x 30 x 19 has 11.6" section width, 25.97" overall diameter, and 3.48" wall heigth

Much more close to the stock rear tire vertical dimensions except it may be too wide for a stock un-widened rim .

 

Discount Tire did mine, I just showed them pics of Gary's car with 285's on them

 

Just to summarize:

It seems that the 285/30R19 is a popular choice for the rear, even tho it is the absolute max you would want to mount on the OEM rim width. This tire is approximately 26" tall (same as OEM tire) and will look great staggered with the OEM 225/40R18 tire up front.

However, if you are also bumping the front tire up to 235/40R18 (which is taller than OEM), you should strongly consider also using a 35 aspect ratio tire in the rear, either 275mm or 285mm wide. I suggest this simply to maintain the original staggered Crossfire look, as a 30 aspect ratio tire in back (at least to me) will then appear too short.

Just my humble opinion.