I have a 1999 base Carrera. I bought some 18” wheels for it. Front are 18x9.5 et 45 cb 71.6 5x130 and the rears are 18x11 et 50 cb 71.6 5x130. I was told by the manufacturer they will fit. Any thoughts on what spacers and tire sizes I will need? Thanks for any help.
If I'm not mistaken, on a base 996, the stock 18" wheel sizes are:
Front: 18x8 ET50 and ran a 225/40/18 tire
Rear: 18x10 ET65 and ran a 285/30/18 tire
The fronts will be sitting about 14mm closer to the strut body and poking out about 24mm more. I don't now how much space there is between the wheel and strut body on a 996, but if it clears the inside, then you won't need a spacer. Otherwise, a 14mm spacer would get you the stock inside clearance, but that would also give you a 38mm poke increase. If you do need spacers to clear the strut body, you might be able to get away with a 5mm or 7mm, as there's usually decent clearance between the tires and strut on a stock setup.
You might get some rubbing at full lock. 24mm more poke is a bit extreme, so under hard cornering, you may rub a bit on the fender edge. Again, I'm not familiar with the clearance of a 996, but rolling the fenders or adding a bit more negative camber (if strut body clearance will allow) might be necessary.
As for front tire size, a 255/35/18 will put you very close to the stock tire size diameter and will be a better match on a 9.5" wide wheel. But you'll also need to take the side wall bulge into account for fitment.
The rears will give you more clearance from your strut body and poke out another 27.7mm more. So you definitely don't need spacers in the rear for clearance issues. Like the fronts, you may need to roll the fenders and add a bit more negative camber to avoid rubbing. I've heard that pre-2001 996 had less clearance than later 996 years, so it's something to consider.
As for rear tire size, a 335/25/18 will also put you close to stock tire diameter, but a 335 is a little wide for an 11" wheel. A 315 would be a better width, but you won't get the sidewall profile you need to keep the stock diameter.
Maybe if you went with a 245/35 front and a 315/30 rear. Your diameters would be slightly smaller, so you'd be doing about 61mph when the speedo reads 60mph. Also a slightly harsher ride with less sidewall. But you wouldn't be upsetting the front to rear diameter ratio. Increasing/decreasing fronts or rears without the other can hinder ABS under hard braking circumstances. It's also an option to continue running the stock 225 fronts and 285 rears, those tires will be cheaper too, but you'll be stretching the tire slightly. Some people like this look, but handling is not as good.
Overall, these new wheels should give you a much more flush stance. Sounds like a pretty aggressive setup compared to stock. If the manufacturer says they'll fit, that's great. But when you start getting close to adding 20+mm of poke and 10+mm on the inside, I get worried that won't fit without some work to the fenders.