I have not heard good things about Nankangs but that was 5 years ago from a guy that worked at his father-in-law's goodyear shop. He tried a set on his mustang and did not like them at all but you can get them cheap on ebay.
All those tire sizes have less than stock heights. You are going to have a larger wheel/fender gap but it should keep them from rubbing, lol. It does look like you are lowered and they look good in your pic. Your speedo will also read that you are going faster than you actually are.
stock 16 225/60/16 - 26.63"
stock 17 (04+) 225/55/17 - 26.74"
your 17s 235/50/17 - 26.25"
your 18s 235/40/18 - 25.4"
new 17 tire 235/45/17 - 25.3" will match with the 18" for now
Next set of tires, try
17" - 245/45/17 - 25.68"
18" - 245/40/18 - 25.71"
On my 04 I found that when I calibrated my speedo (with a PT by changing pulses per mile) for tires at 25.66" my cruise control wouldn't stay on for more than a few minutes. I posted the problem on the old club and JohnH told me that the maximum pulses per mile was 24100 ppm to keep the CC happy. I found the post again, thanks to JohnH on the old club who apparently now has a G8 GT!
here's a link
http://www.clubgp.com/newforum/tm.asp?m ... s=#4540152
Stock pulses per mile is 23430 ppm, a 275/40/17 is 24419 ppm. The maxim that John found was 24100 ppm. A 255/45.17 is 24068 ppm. Cruise became happy again! I don't know if these values apply to a 97-03, hopefully not.
In your case a 235/45/17 is 24636 ppm and a 235/40/18 is 24563 ppm. Since the maxim is 24100 ppm you may have to live with a slight speedo error, maybe 2 or 3 at 60mph even if you adjust it with a PT. If your next set are 245/45/17s (24296 ppm) and 245/40/18s (24262 ppm). Since the maxim is 24100 ppm you may still have to live with a slight speedo error, maybe a mile or 2 at 60mph if you adjust it with a PT. Or you could just live with a little bit higher than that if you don't have access to a PT. Your speedo will read that you are going faster than you actually are. Larger height tires will make your speedo read that you are going slower than you actually are, better chance of getting tickets that way, lol. One thing to consider is that your tranny shifts according to mph and rpm based on whether or not you are at WOT. If you only changed your pulses per mile your shifting will be freaked out a bit. If you don't change ppm your shifting will be like it always has been according to rpms and your incorrect speedo. So if you don't want to have to change your mph shifting tables you could just skip fixing the speedo calibration with a pt for now and shred your tires every chance you get! Next set you can have more choices.
I've run
275/40/17 - 25.66"
255/45/17 - 26.04"
without rubbing, these were on the bonneville wheel pics in my sig, I'm not lowered, yet. I had huge wheel gap with the 275s. The 04+ cars have a larger wheel well it seems.
I just purchased some new Enkei wheels and new tires yesterday. I'm hoping to get them Friday. They are guaranteed to fit before I lower it but I'm not sure what will happen after. Probably rolling the fenders, ha!
The wheels and tires are
8.5" x 18" with a 35mm offset
245/45/18 - 26.68"
They are pretty close to stock 04+ size 26.74
I may try 255/45/18s next.
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39# inj, LQ4, FWI, Jardine exh, OBX hdrs, PT, AL104s, PRJ Wires, DEI boots, 180 stat, 3.4 MPS, MSP poly eng mnts, F&R STBs w/ DR brkts, poly sway bar bushings, painted tails, SPO grills, spoiler & tips
But why are they different sizes front & rear? 
You need to match the fronts with the same diameter as the rears with this tire calculator.
