Converting Slick Sizes to Street Sizes
 

Hello mates. I have a set of hoosier slick that need changing and I may go the DOT route. How do I convert the sizes this is what I havee:

Front - 250/650R18
Back - 305-645R18

Thank you in advance.

250 width in mm
650 tire height in mm
18 wheel diameter

closest street R tire to a 250/650 x18 is 245/40 x18
closest street R tire to 305/645 x18 is 305/30 x18

Slicks measure the sidewall as total height, so a 650 (Hoosier) or 65 (Michelin) slick is 650mm in diameter, converted to inches, 25.59". A 645mm is 25.39"

In regards to width, each tire brand/model has variances, so it is better to use the factory specs.

The 250/650R18 Hoosier R80/R100 has a width of 9.8" on a 9" wheel with an unloaded contact patch (tread width) of 9.1". The closest DOT Hoosier R6/A6 to that size is the 245/40R18.

The 305/645R18 R80/R100 has a width of 12.7" on a 12" wheel with an unloaded contact patch (tread width) of 11.8". The closest DOT Hoosier R6/A6 to that size is the 315/30R18, almost an exact match at 12.5" total width on a 12" wheel with a 11.8" tread width.

Generally, the ideal setup track tire for the 996 GT3 street car on 9"/11" or 9"/12" wheels is the 255/35R18 and 315/30R18. I like short profile tires on the front axle due to the more precise steering, and I like tall profile tires on the rear due to the heavy load rating on taller tires.

I would go 255/35R18 and 305/30R18 R6, long lasting tires (16 to 20HC, some guys get 24HC), affordable, and plenty of contingency programs in NASA, SCCA and NARRA Club Racing and Time Trials to re-stock free tires.

I also prefer the 245/35 to the 245/40 in a 996 GT3, but given the existence of the nice 255/35, this is my preferred tire from the 3 Hoosier sizes. If the car is a little too loose, just stiffen the front sway bar one hole, and back to neutral.

Thank you so much for the all the info!

I'm considering BFG g-force. I'm told the may take a little longer to get to temp but that they are good and can get over 30HC in DE. Here is there size chart...would any of these work? I'm thinking 245/40 and 285/30??? Thoughts.

225/40ZR18
245/40ZR18
265/35ZR18
275/35ZR18
285/30ZR18
285/30ZR18
335/30ZR18
335/30ZR18

we run BFG R1's 245/285 (f/r) on SPEC996's. they are good for racing to 20HC, and practice or DE until 35-38HC. The harder you push them, the better they get, but I doubt you'll out run your early cycle Hoosier times.

+1

To answer your question several points need to be addressed
1) stock abs?
2) stock abs wheel tooth count?
3) did the previous tires, I assume Front - 250/650R18,Back - 305-645R18, fit & provide the gearing & work w/ the existing abs to your complete satisfaction?

stock abs. The previous hoosiers were awesome and worked fine. I decided to get a set of hoosiers and a set of bfg and test them on the same day side by side. I'm okay giving up 2 seconds if I can get more time on the tires. I'm shredding the hoosier and driving them to cord which is way past it primee the hoosiers for racing.

stock ABS works best when the front and rear tires heights are the same(there is some wiggle room but the further from 0 deviation the better), your old set up using 650/645 had a deviation of less than 1% which is good

you don't mention wheel widths but assuming 9&11 or 9&12 wheels Hoosier R6 has either a 315/30 @25.6" tall or a 295/30 @25.3" tall a 255/35 front @24.8" tall gives a bit less than 2% delta w/ the 315/30 and is also acceptable

BFG unfortunately doesn't have the Hoosier sizes, the best they can do is a 285/30 rear(too small) or a 335/30 which is too tall and needs a 12" wheel unless you use a 265/35 front

This was posted in a different thread as well, but the BFG's run VERY wide for their advertised size. Matter of fact, NASA has assessed extra mod points to the BFG tires not becaus of their speed but due to how large they run.

I used to use 265 BFG's on my M3 and they were as wide or wider than the 275 Nitto's I used on that car.

Spec's for the Hoosier vs. BFG

Hoosier: 315/30/18 245/40/18 **255/35/18**
Section width: 12.5 10.5 10.8
Tread Width: 11.8 9.3 10.3
Diameter: 25.6 25.7 24.8

BFG: 285/30/18 245/40/18
Section width 11.4 9.8
Tread width 11.5 9.5
Diameter 24.9 25.4

Added the 255 R6 because it's a popular alternative front tire many of the '6 GT3 crowd have gone to.

Hi guys,

I've bought a 993 RSR replica on slicks and I'm going to switch to DOT, to drive to and from the track.

https://dl.dropboxusercontent.com/u/...2013032242.jpg

The slicks are Michelin 24/64-18 and 27/65-18. They fill the arches perfectly, so I'm keen to get equivalent street sizes.

The rims are 8.5" front and 10" rear.

For the front, I'm thinking:
235/40-18 (Pilot Sport Cup or Pilot Super Sport)
For the rear, I'm thinking:
285/30-18 (Pilot Sport Cup) or 285/35-18 (Pilot Super Sport)

I prefer Super Sports, for their much better resistance to aquaplaning.

Would you please help?

Thanks,

Merely curious here, but what clutch package is in the RSR - and how does one keep it alive on the street?

It's just an RSR replica, based on a regular 993 and then profoundly modified. Clutch is std.
To drive on the road it just needs DOT tires, H&R springs, a horn and licence plates. :-)

That car is beautiful... Very nice! :) :)