Is 29 extra revs/mi OK for 4S AWD?

[dimsdale]

TireRack sold me a winter setup which is on the car now, and I just did the math and I'm not sure about it. Thought I'd check in here. The revs/mile different between the front and rear for the TireRack setup are twice that of the factory recommended setup.

TireRack sold me this:

Rear: 265/40/18, which has revs/mi of 765

Front: 235/40/18, which as revs/mi of 794

Difference: 29 revs/mi or 3.79% more revs/mi on the front.

The Porsche recommended setup for winter tires is:

Rear: 295/30/19, which has revs/mi of 777

Front: 235/35/19, which has revs/mi of 792.

Difference: 15 revs/mi or 1.93% more revs/mi on the front.

They swear this is OK. Obviously, my concern is the center diff. Anyone have any opinions on this?

Many thanks in advance.

[RFM]

The tyre dimensions for 18" and 19" are the same as in the manual for 997. Looks good so far.

[dimsdale]

The tyre dimensions for 18" and 19" are the same as in the manual for 997. Looks good so far.

The dimensions are only correct if you use the 2/2S specifications.

The manual says the 2S can have

front 235/40 R 18 and rear 265/40 R 18 <-this is what I have from TireRack

or

front 235/35 R 19 and rear 295/30 R 19

The manual says that the 4/4S needs

front 235/40 R 18 and rear 295/35 R 18

or

front 235/35 R 19 and rear 295/30 R 19

Doing the math, the 4/4S setup has a closer revs/mile than the 2/2S setups. The fact that Porsche explicitly recommends my TireRack setup for the 2/2S and NOT for the 4/4S gives me some concern.

Edited February 11, 2009 by dimsdale

[RFM]

What i have from the manual: summer 235/40 ZR 18 on 8J x 18 ET 57 - 265/40 ZR 18 on 10J x 18 ET 58 // 235/35 ZR 19 on 8J x 19 ET 57 - 295/30 ZR 19 on 11J x 19 ET 71.

winter: 235/40 R 18 on 8J x 18 ET 57 - 265/40 R 18 on 10J x 18 ET 58.

Dated 04 - 2004 for European cars, could be different for US and already overhauling.