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Well yeah, just showing the OP which tire size of preferred. Obviously in the real world you can have fronts with less thread depth, different psi to throw off the % but as long as you are in the ballpark, you will be fine.There were theories why the rear should be slightly smaller then front but I think those were busted. Without looking up the exact numbers, the Porsche summer tire recommendation had a slightly smaller rear diameter, while the winter recommended sizes have a slightly smaller front. I don't think it really matters which one is smaller as long as they stay within a certain % difference.
I jack up my 996tt by the rear side jacking point first (close to rear wheel). this lifts up the entire side of the car. I then place the stand in the font jacking point and lower the car. Repeat on opposite side. Then at the end I jack up the rear by the cross bar in the back, not the engine and add stands under the read side jacking points.
There is a % difference the front and rear wheel diameter needs to fall within so the VC stays happy. I forget but it is something like 3% (don't quote me on this % number and please research what the acceptable % difference needs to be) Your proposed setup 235/40-19 = 26.4" 295/30-19 = 25.97" % difference = 1.64% My setup 245/35-19 = 25.75" 295/30-19 = 25.97" % difference = .85% You want to be as close to zero as possible. If I remember correctly, Porsche's summer and winter tires recommendations all fall under 1% in diameter difference.