Acceptable rolling diameter differences between front and rear tires

[Lemikson]

A new owner of 02 996tt with a question about the tires and acceptable rolling diameter differences between the front and rears. I apologize for bringing this question again, did an extensive search here and on rennlist and I am finding a bit of a conflicting information.

 

So far, looks I have come to conclusion that the front to rear tire diameter difference within 2% is predominantly acceptable by forum users that appear to be knowledgeable and factual. Some of them are actually stating to stay within 1% max.

But, then I ran into a replay to similar post from a very reputable member who is claiming “as a rule of thumb difference up to 4% is acceptable”!!!.

 

One of the explanations for 1% or less that I have found is that the original P-design setup was with 3 revolutions per mile difference (832 @ R and 829 @ F) which is 0.3 % diameter difference (616 mm (24 ¼”) R (~ 24 3/8” ) and 618  mm F) – almost identical rolling diameters between the axles. I did not have a chance to check for available information in P technical publications to find any design/engineering rational for such interpretation of the original set up. 

Now, if you allow for 2% diameter difference you are ending up with almost 7 times as much spin differential in the VC, not to mention 13 times as much at 4% diameter difference. 4% also sounds a bit large as the axle center would go up for almost a half inch with the tire diameter increased for 1 inch one over the other (4% at 25.5” reference diameter).

 

I might be wrong on all of this, and I again I did not have a chance to check in P technical publications. I apologize on a long note and appreciate any clarifications and advice as my plan is to try to come up with some out-of-ordinary tire size combinations within the 18`OE rim constraints (strictly driven by limited choices I have here in Canada, plan to control my budget and idea to get started with a good ``starter`` street tire that is not Michelin PS2  :D and slowly to start learning the car  - would like to go with Conti DW or Super Sports but struggling to figure out correct sizes ). 

 

Thanks.

[wross996TT]

Of course the closer to "identical" rolling diameters the better.  That being said, most folks are talking theoretical ratios anyway.  I mean seldom does anyone actually measure the rolling diameter of a tire and going by the tire size is completely theoretical (I am sure tire diameters vary).  Best to actually measure the tires and go from there.  I believe the "published" acceptable theoretical difference is 3% FWIW.

[DaveCarrera4S]

There was a lot of discussion on this subject over at 6speed.  the upshot is that the front tires should be slightly larger than rear.  This keeps power flowing to the front, and it was stated that this is how the car came from the factory.  (Can someone confirm factory tire sizes??)  If the rear tires are larger diameter, torque flows from the front, or causes a slight drag on the VC.  None of this is likely to cause any problem to the VC.  So in the end, try to have the fronts slightly larger than the rears.