Any good guidance on how much variance is tolerable between front and rear?

235/35 and 305/30 19s are almost a full inch in diameter difference

GT2 fitment 235/35 and 325/30/19s are 1.2" difference in diameter.

 

don't look at inch look at % 235/35 and 305/19 difference 2.9% ...... 235/35 and 325/30 difference 4.7% not very good for 997tt

 

Also remember GT2s are 2WD (RWD). I would not go over a couple% difference.

DC

 

The front and rear final drive gears are 3.3% different. So you have some leeway, maybe .5% for tire wear. The front drive clutch can allow some slip, running in sport mode also engages the front drive much less.

 

Keep in mind the front and rear gear ratios are different in the 997 Turbo. Most people forget about this.

Final drive - rear 3.44
Final drive - front 3.33

Or about 3.3% difference. You actually need a 3.3% taller tire in the rear to make them perfect.

Co-incidence that Porsche uses a 305/30/19 and a 235/35/19 from the factory? That's a 2.7% difference and is on the edge of what most people recommend - but in reality it's only .6% when you account for the final drive ratio differences.

GT2 style fitment of 325/30/19 and 245/35/19 have a 3.5% size differential, however once you subtract the final drive differences you end up with an effective .2%. difference. That is as close as it gets ...