My 4th of July holiday project was upgrading the tires on my Z. I spent quite a bit of time researching my options, figured I might as well share the experience here...
First consideration, the issue of Active Handling, ABS and Traction Control. These systems use wheel speed to make critical decisions about actions they should take. For example, comparing front and rear wheel speeds can tell the TC that the rear wheels are spinning, or tell ABS that one wheel is headed for lockup.
The front and rear tire diameters of a Z06 are different. Factory sizes are 26.13" rear and 25.35" front. If you change tire sizes, it's important to maintain the same ratio (or as close to it as possible) between front and rear tire diameters. Too much change confuses the computer systems and can negatively effect TC, ABS, and AH.
A thorough search of this Forum provided enough information to conclude that changes of more than 2% in the ratio can be problematic. Symptoms of "bad" choices in tire size changes mostly included reports of early onset of Traction Control under heavy throttle, but there was also some mention of questionable ABS and/or AH behavior. Some posters had to turn off AH/TC for any spirited driving activity.
I decided to work towards a budget of less than 0.5% to be sure I'd minimize the risk of any problems with the computer systems.
Next, I needed to know how wide of a tire would fit in the rear wheel well without rubbing. It's pretty clear from existing Posts here that 345s are too wide and need a tubbed wheel well. It's not so clear if 335s are OK or not, "minor rubbing" was used to describe the fit by a few posters. IMO, no amount of rubbing is acceptable, so I opted for 315s.
For 18" rims, 315s are available in 40 and 30 profile sidewalls. 40 profile are nearly 7% larger in diameter than the 295s. I was worried about wider *and* taller tires not fitting, so 40 was ruled out in favor of 30.
A 315/30-18 is 97.4% of the original 295's diamater (2.6% smaller). Pretty close, but over my 2% front/rear delta budget. I could have a problem with these rear tires if no additional changes were made up front. I decided I needed a front tire that was at least 0.6% smaller, and ideally closer to 2% smaller than the 265, in order to match the change I intended to make to the rears.
Unfortunately (or not, depending on how you view it) there really aren't many(any) wider-than-265 tires for a 17" rim with diameters that come close 2% smaller than the original. However, if you go up to 18", there are many options.
Ultimately, I chose a 285/30-18 with an O.D. of 24.73", or 97.6% (2.4% smaller) of the OEM 265's diameter. I needed 18" front rims, but with this tire the delta between front/rear ratio is only 0.2%. Normal tread wear changes O.D. diameter by that much, so I was confident this would work without upsetting the computers (probably, indistinguishable from OEM).
I couldn't find any definitive answer in the Forum about whether or not a 285 would fit up front. The 285 is 20mm, or just as little less than 1" wider than the 265. So I figured that as long as there was more than 1/2 inch of clearance either side of the 265/40-17, a 285/30-18 (with it's slightly smaller diameter) should work OK. So, 18" rims all the way around.
I decided not to go for a more "exotic" 19" or 20" wheel. Generally, quite a bit more expensive, and fewer tire choices in wider widths.
The best looking tires (yes, look is important to me) in these sizes were the Michelin Pilot Sports with a 315/30-18 and a 285/30-18. These are supposedly the OEM tire for a the Carrera.
To go with the tires, I bought a set of Chrome Z06 Motorsport wheels from Xtreme, 4x18"x10.5". I opted for a 10.5" front instead of a 9.5", since Michelin says 10" is nominal for the 285 tire and 10.5" is nominal for the 315. (in retrospect, I might have gone with the 9.5" up front, anyway. I don't like the way the tire sidewall looks on the rim up front, as much as I do the rear. And, I think the front could use a slight inward offset, which the 9.5" probably had).
My local neighborhood Chevy service center kindly provided me with a set of 4 tire pressure sensor/valve stem assemblies and the required outer securing nuts for a *very* reasonable cost! I was worried about fit, but was pleased to see they mated up to the Motorsport wheels perfectly. Note: The Sensors do NOT come with nuts, you need to order 'em seperate. Also, the '00 and later part is different from the '97-99 version. It's quite a bit smaller, but it has a *much* larger O-Ring. I don't know if the radio frequencies are compatible, but the O-Ring is not. The older size sensors will NOT seal at the valvlestem hole of the Motorsport wheel (I have an old sensor from my 98 Coupe, and I tried it). If you buy sensors, make sure it's for the right model year!
My friendly local Chevy service manager also kindly enabled the Z06's option-delete TPM system using their Tech-II scanner (had to listen to it bitch "Service Tire Pressure Monitor" on the DIC for a day, before all the other stuff showed up). He also lent me the "Training Tool", which is really just a big magnet. You need this SST in order to teach the computer which wheel is which (kind of like training the Key Fob).
I ordered Tires and Wheels on Monday the 30th and it had all been delivered to my front door by 2:00pm Thursday afternoon. Left me just enough time to get it all put together at the local speed shop, before everyone went home for the long weekend.
Ha! I love it when a plan comes together!
I'm pleased to report that everything fits fine. Clearances both up front and in back are close, but I have not been able to get anything to rub, even on hard corners and/or bumps, even with with a passenger in the car. The suspension on my car *was* lowered the factory-bolt maximum-amount a few months ago.
The Tire Pressure Monitor works great, exactly like it did on my 98 Coupe. There's quite a lot to be said for being able to check each tire's pressure whenever you want, from inside the car, even while driving down the highway! You may not know what you're missing if you've never had it, but of all the "convenience feature" differences between my 98 Coupe and the Z06, I missed this most of all.
The Michelin tires, mounted on the Motorsport rims, are noticably heavier than the F1s on the OEM wheels. Some of this is the increased rubber, but I suspect most of it is the repro wheel. I did not measure the difference, but I don't expect to drive in a way that it could matter much, anyway. Just an observation, the OEM wheels are pretty light weight.
The best news is that the Pilots look *great* on the car, have *tons* more grip than my F1s did (they were pretty worn out, though, that last 1000 or so miles), and are substantially quieter on the highway.
Here's a pointer to a handy Spreadsheet you can use to do the math yourself.
Tire Calculation Spreadsheet
Here are some pictures (click for larger image):
The air pressure sensors were never on my list of mods... until now. Great piece of mind.
This is a wonderful spreadsheet. I have started this study a couple of times and kept deciding to just leave it stock. 
Wally
Linear Mode
