Why is my rear tire smaller Diameter than my Front ( ? )

Bill, John Ellis from GGR San Carlos checking in again.  I will send some images to the PCA.org address to better demonstrate my challenge/ question.  Here is what's up.  I used to have a 2001 Carrera with 19" Lowenhart 3 piece wheels, Rear tire size is 285-30-19 ( 25.7" diameter ).  I have no clue what the offset is as I can't find much info on these nice wheels as Lowenhart is out of business. Regardless, I really like the appearence, fit and performance of these wheels ( my tires are the Conti DWS fitment ). A liitte inside rub on the fronts, but nothing extreme or damaging. This car was totalled in Mar, yet I kept these wheels.

Now I have a 2004 996 with the factor 18" hollow spoke Turbo wheel. The rear ( 18" 10J 65mm offset ) with Sumitomo HTRZ 285-30-18's just don't seem to fill out the wheel well as much as the should ? The fronts ( 225-40-18 ) seen fine.  The other day, I measured with a tape and see the fronts are 1" taller than the rears ?  I know the 285 -30s are wider, yet they seem to leave a lot of open space in the Wheel well. I looked at going to the 295-30- 18s that are recommended for the turbo, yet the turbo wheel is an 18 -11J  albeit this tire is not acceptable ( per tirerack ) on the 10J wheels.

All this is simply aesthetic as the car handles and performs great. My question ( finallly ) is what's up with Spacers ?  I've done a little reading on them and appears something between 12 -17mm helps push the tire-wheel a little further out.  Is this really worth the effort ? What would a 12- 17 mm movement of my wheel-tire do to the suspension geometry the factory set the current wheel tire placement ?  Or just install my Lowenhart set up ( this model were made specifially for the 996 no spacers ) and call it a day.

In summary to Spacer or not to keep the Turbo hollow spoke,  if so what's the recommend least disruptive size/s ?  Thanks for all you do for our club, always appreciate your subject matter expertise and insights.  Cheers,  John Ellis ( john_ellis@hermanmiller.com )