Tyre Pressures for 19" wheels?

Hi All New to this forum as only just purchased my first 911. It's a 1998 996 tiptronic but is fitted with 235/35/19 on the front and 265/35/19 at the rear. As these are not standard or optional sizes there isn't any official guidance on pressures. Had one garage say 36psi all round and another says use the same as for 18" being 36psi front and 44psi rear. The car won't be used for any high speed stuff on the track. Has anyone any experience of these sizes so I am both safe and also have them wearing well? Thanks.

Steve! Same size tyres as used on the Gen1 Caymans, like mine. For the Cayman, they specify 32psi front, and 36psi rear. Both are about 1350kg, so I think that 32/36 would be a good starting point.

What so 997s get filled to? I'm just thinking that as 17 to 18 gets an increase in the rear surely going to 19s should increase again?

Thanks Richard I'll try the tyres at the lower pressures, should at least make the ride a bit softer. Quite a difference between 44 and 36 at the rear!

Track tested my car, albeit on 18 inch rims and found the best was 32 psi when hot, so subsequently nitrogen filled (No variation when hot) the tyres to 32 psi all round and it has been excellent...certainly better than the higher settings which cause the steering to pull on bumps..although if I was going to be doing some serious 100 mph plus cruising on the autobahns, I would put them up to 36 all round. Hope this helps?

Thanks Chris/Stuart Quite why it jumps from 36 to 44 between the 17" and 18" beats me, it seems to be a very large jump but I don't think I should be going higher than 44psi at the rear, presumably its all to do with the depth of the tyre and weights involved. I have been pondering this quite a bit. Wondering that if there is a difference in weight between rear and front then should the rear then have a higher pressure to compensate? Also as these are fairly low 35% profile tyres then to avoid damage in potholes etc should the pressures be higher than say on my Focus which are 33psi but have much deeper bit of rubber between the alloys and the road?

235/35/19 is also the front tyre size for the 997, and they specify 33psi partially loaded and 36psi fully loaded. (Cold). The 997 uses much wider and lower profile rears at 295/30/19 and 39psi partial load and 44psi fully loaded. I think that 44psi will be too high for the rears, and you will get more wear on the centre of the tyre.

Thanks Richard I'm definitely overthinking this one! As far as I can tell the 996 is a bit lighter lighter than the 997 with more or less the same weight distribution. So the front tyre pressure of around 33 psi or slightly less seem about right. The 997 tyre being 295 x 30% must be 88mm deep and mine must be 92mm (265 x 35%) which is pretty close. So again perhaps a little lower than the 39psi so this puts it back to around the 32/36 that you originally suggested, although against this the weight distribution of the Cayman means more weight on the front so getting myself well and truly confused trying to work this one out, you'd think there would be some formula somewhere!

I have no idea where one would start to calculate the tyre pressure for a specific tyre size. There are so many variables to the calculation - width, profile, diameter, horizontal and lateral forces, weights on each axle, etc. Edit: a starting point might be just to compare axle weights and base it on that. Both Cayman and C2 are about the same weight, but the C2 has a weight dirtribution of 38-62% F-R, and the Cayman is 45-55%. Although I guess ifyou did that, you would be running the fronts at less than 30psi, which wouldn't seem right to me.........

Thanks for all the replies. I will decide on some pressures and run them for a while and see how they work for me. I'll come back to this post in a few months and post the pressures I've been using and how they worked for me both in terms of handling and wear (if I can tell by then). This may help anyone else who has the problem. Guess I should have got a 996 with standard sizes! But the 19" do look nice!

Unless you emailed porsche and see if they had any rough guidelines.

Found this...... http://www.uklegacy.com/forums/index.php/topic/98525-calculate-correct-tyre-pressures/ There seem to be a few apps for doing the calcs as well - might not be 100 percent but its probably a starting point?

Thanks for the suggestions. I'll email Porsche and also take a look at the formula when I get a chance to look at the load rating of the tyres. I will check the accuracy of using the formula by comparing it with the standard advised pressures on my other car and see if it agrees!