My car is a 996GT3 and I am looking at a few sizes of various diameters and looking at the diamter of the various tires knowing that 25" is the target for the factory suggested diamter, what effect would there be on the following setup possibilities? I understand the rec'd variance of <4% which all these fall within but it is the actual effect they have I am interested in. (speed, speedo, gearing, sidewall, etc.)

Front 24.4"
Front 25.3"
Front 25.6"

Rear 24.6"
Rear 25.3"
Rear 25.4"


TIA!

 

It's all percentages. If a given size is 3% bigger, for a given speed the motor will rev 3% lower, the speedo will read 3% lower than true, sidewall will be 1.5% taller in radius (3% in diameter).

 

so would you prefer to to go a few tenths above or below the rec'd 25" ID? Adv/Dis by choosing one over the other?

 

What are the dimensions and brands of the three tire sets?

 

its not all about straight percentages, don't forget the difference between the front and back wheels in terms of revolutions per mile. Worst case difference in the examples quoted here is 26 revolutions.
Best thing to do is measure the circumference of the production tyres when new and fitted to the car, and work to duplicating that difference as nearly as possible, but do not reverse that difference. P-cars have a huge and underused rear contact patch from a braking point of view, unloess one is driving backwards, that is.

R+C

 

Hoosier R6

The fronts:
245/35/18 with an ID of 24.4"

The rears:
285/30/18 with an ID of 24.6"

 

Ask Larry. Alot the guys in the club that run R6's on GT3s use 245/40 fr, 295/30 rear.

 

Thanks bob. I'll get in touch with LH. I am thinking about both the ones I posted and the ones you just posted. Just trying to figure out really what the differences are between the 2.

 

I thought you were going to Slicks Sandman?

 

Been giving it alot of thought and decided to move to the R6 for a period of time. Main reason right now is suspension bits needed to run them "properly". That is an expense I am not in the mood to do just yet. Could run them as is but probably not safe. I'm in no rush anyway. Besides, it's will keep the dissenters off my ***