Russell's Tech Tips

Upgrading Wheels on 930's

Bill Hawe

Bill930@PorscheNet.com

The Stock Setup

The stock 930 (through 1989) uses 9x16 Fuchs on the rear and 7x16 on the front. It's tires are 245x16's on the rear and 205x16's on the front. Often it is desired that the wheels be upgraded to larger sizes. One popular upgrade is to use 9x17 Ruf wheels on the front and 10x17 on the rear. With this upgrade the rear studs need to be replaced. The Ruf setup uses a narrower rear spacer than the stock setup. Thus the new studs will be shorter than the stock studs. Popular tire sizes with the Ruf setup are 235x17 on the 9's in the front and 255x17 or 275x17 on the 10's in the rear.

Note that wheel width is specified as the distance between the points of contact of the tire beads on the wheel, not the outside dimension. So, typically a 9x17 wheel measures out to be about 10 inches wide if measured on the outside to outside dimensions. It is virtually impossible to measure the bead-to-bead dimension of the wheel with a tire mounted on it.

Track Applications

For track applications it is common to want more rubber on the road. One popular setup is to put the 9x16 Fuchs on the front and put larger 10x17 or 11x17 on the rear. This is often done for Yokohama tires since Yoko doesn't make a moderately wide, sticky 17" tire. Thus with 9x16 on the front and 10x17 or 11x17 on the rear one can use Yoko 245x16's on the front and 315x17 on the rear. The Yoko track rubber will fit a 10x17 but that size rim is the narrowest that can be used. In reality, the tire is too wide for that rim. It should have an 11x17.

If one uses BFGoodrich R1's then there are more options in sizes for sticky rubber. 9x17 can be used on the front with BFG R1 255x17's. BFG makes a 315x17 R1 which can be used on a 10x17 on the rear. However, the tire works better on an 11x17 on the rear. If you mount the 315x17 BFG R1 on a 10x17 Ruf rim you cannot use the Ruf spacers. The tires will rub on the fender lip. The 315 is going to be a little large for the 10x17 rim. Thus it rubs. Using a 5/16" spacer will move the wheel/tire inboard enough to eliminate the fender lip rubbing. However, when you do this, if you still are using the Ruf studs they will stick out too far to use the stock lug nuts. Measure the inside depth of the lug nuts carefully and you'll discover this. Therefore, in this configuration one must use open steel lug nuts to account for the (relatively) longer stud length. So, 315 on 10x17 Ruf with 5/16" spacer works. But, steel lugs must be used.

I have heard rumors of using a 12x17 on the rear with 315's. However, I have not actually fit one so I'm not convinced it works. In fitting the 11x17 it looked like, with care taken as to the offsets, a 11.5x17 would fit ok.

A few comments about the front setup are in order. Using a 255x17 BFG R1 on a 9x17 does work fine. However, the inside of the fender lip will most likely need to be "rolled back" somewhat. I have also had the inside of the fender lip trimmed to make it narrower. This all works fine with no rubbing (on my car). You may also evaluate the proper camber to use as this will help with clearance. Other "255's" may not fit the same way though. You need to obtain the tire specifications for cross section, diameter, etc.. Compare these with the BFG R1 255/17. No doubt you'll notice a small difference. In effect, all "255's" are not created equal. You may find that with your tire brand a 245x17 or a 235x17 is what fits.