The stock tires are the same diameter, 24.9", so *ideally*, you want fronts and rears to be close to the same diameter (more important than they're being as close as possible to stock diameter).

245/45-16's are 24.6" (every one I looked at, anyway).
205/50-16's are 24.1".

If you go with 245/45-16 rears, you can go with 205/55-16 fronts, and have the fronts be 0.3" bigger in diameter than the rears, or go with 205/50-16 fronts and have the fronts 0.5" smaller.

The main concern with having different diameters front/rear (when the car is designed for equal diameters) would be the change in brake bias. Smaller rears biases the rears more, smaller fronts biases the fronts more.

FWIW, I drove on 205/45-16 fronts, 225/50-16 rears at Mont Tremblant earlier this year (not my ideal tire size choices, tires were given to me). 23.2" front, 24.9" rear, fronts a significant 7% smaller in diameter than the rears. It did seem to me that the fronts were doing more than their fair share under braking, and that the rears weren't doing as much. Not ideal, but not really a huge problem either (passing only on the straights, not racing, getting "outbraked" wasn't a concern). I beat my performance benchmark (former club champion in his 350Z) in the time trials, I don't think I was giving away much in lap times

That's a pretty extreme example of front/rear size differences. My Summer street tires were 205/50-16 245/45-16, 24.1", 24.6", fronts 2% smaller than rears. I drove on these at Lime Rock and didn't note any brake bias issues.

If you're going with 245/45-16 rears, you *could* be giving away something between little and nothing in outright braking performance going with 205/50-16 fronts (2% smaller fronts, slightly more front brake bias) vs. going with 205/55-16 fronts (1% bigger fronts, very slightly more rear bias), assuming the stock brakes bias the fronts as they should. I SERIOUSLY doubt this difference would even be within the range of error should a test between the two setups be conducted.

My *opinion* is that the 205/55-16 fronts appear a bit econo-sedanish, and even with same-diameter 225/50-16s in back, the fronts look tall (due to the raked styling of the car) relative to the rears. With shorter tires in back, the "tall front tires" look is quite apparent, and not at all appealing (to me). Shorter stiffer front sidewalls didn't seem like a bad thing to me either (though it's possible the uninitiated might get caught out by theoretically ever-so-slightly greater turn-in response), so I went with 205/50-16s up front 245/45-16 rears for my Summer tire sizes. Looks bitchin'. Worked great on the street and at the track.

Your mileage may vary...

 

Thanks, I will get more info and post it. ZDan, you sound happy with your "Summer tires", What were they and where did you get them?
Steve

 

Hankook Z212 RS2's, from edgeracing.com

Wet and dry performance is excellent, but life won't be much if any better than OEM. I'm at 9k, looking to get ~15k out of them.

Great "max performance" tires, particularly for the price.

 

Just put 4 BFG g-Force KDW2's on my '05 (first set of replacement tires, the original Bridgestones lasted 18K miles). I was concerned about potential noise issues with them, but that just simply isn't an issue! They aren't noticeably louder than the stock tires, although the grip has gone up exponentially. Costco was the source.

 

mloomis,
What size did you go with and was it the right choice?
Steve

 

I went with the absolute stock sizes for my '05: 215/45-17 for the front, and 245/40-17 for the rear. Based on how the car handles now, I can't see how any different size could be an improvement. No fuss, no muss, just corners precisely and predictably with tremendous grip.
Mark