xmaciek82x

Bare with me since I'm new to Porsche. But a regular non-turbo Carrera wheels bolt right up to a 996tt? I'm assuming your ten inch wheels came off of a narrow body Carrera.

993GT

wrong offsets for a TT/widebody.
Could try 18" 993 widebody wheels which are 10" rear..18x10" et40, add a 10mm spacer to be nice and flush

Carlo_Carrera

They are an older set of Giovanni wheels. I do not know much about them. They fit fine without spacers. They and a set of stock turbo twists came with the car when I bought it.

xmaciek82x

I never realized that 993 turbo came with twist wheels. I thought it was a 996+ up wheel. That is good to know. So would a 275/35/18 tire on a 10" wheel work with a front 225/40/18 on a 8" wheel?

993GT

Better to match a 215/45 front(good on 8" wheels) with a 275/35-18 rear. 275 is happy on 10" wheels
993 wheels are a different, smoother Twist design...any 8/10" wheel combo for a C4S/TT 993 will be good

Carlo_Carrera

Not really. The overall diameter of the rears are too large.

I run 19" winter tires in 235/35 and 265/30. But those sizes are not available in 18".

Others here can chime in with proper 18" sizes.

993GT

a 275/35-18 has almost the same diameter as a stock GT2 315/30-18, no issue...

Carlo_Carrera

It isn't a fitment problem. It is a rotation per mile problem with regards to the AWD system. If it possible you always want the rears to be a slightly smaller diameter than the fronts.

You can use a 275/35 but you have to pair it with a larger diameter tire on the front.

993GT

Yes, that's why you match it with a 215/45-18 front tire. You want a larger front tire so the VC is over-driven by the quicker spinning rears and allow the front to yield useable torque

Carlo_Carrera

Yes, exactly. You just shouldn't pair a 275/35 with a 225/40 in the front like he was asking.

993GT

Agreed. A 255/35-18 could work with the 225/40-18 front and still stretch onto a 10" wheel, but not ideal.

breakfast

Porsche officially specs for winter:
225/40/18
265/35/18

that rear is slightly bigger than the front(but still within reasonable limits).

F 25.09" vs R 25.30 "

I run F225/40/18 and R 255/35/18 (on 10" 993 twist replica wheels). 255's are good with 8.5-10" wheels.
The 255 is 25.03" which is ever so slightly smaller than the front, but again, still within spec.
It doesnt really matter either way unless you are talking 3+% difference.
Even then it would still technically function.

xmaciek82x

Well that was helpful! Thanks guys. Does the rear NEED to be smaller because of the awd, our can both front and rear be the same? I know in my Evo I always had to match front with rear because of the constant 50:50 power distribution of the awd system. I'm guessing the 996tt always has more power in the rear, therefore a smaller rear tire diameter is required.

breakfast

It doesnt matter as long as you are in the ballpark.

The oem size for "summer" has the rear being slightly smaller than the front, but the official porsche spec for winter, the rear is slightly bigger(differences in fractions of an inch).

Carlo_Carrera

Most Porsche techs I know say absolutely the rears should be slightly smaller or the viscous coupling starts working in reverse causing it to attempt to slow down the rears.