jhbrennan

997.1 for both.

L_perm

I knew Porsche was going to come up as the manufacturer to listen to when I wrote that. Stepped right into that one.

I'm not claiming I'm right. I'm arguing points of debate that IMO are relevant to Bob's situation:

1) Do the tire manufacturer's recommended design range of rim widths matter to Bob? I don't know, but it's something he might want to consider.

2) Is there a potential downside to 305's on 11.5" or 12"? Less protection from potholes and curbs? Seems logical. Michelin verified my thinking. Should Bob consider this? It's up to him, but I thought it worth mentioning, particularly since he was asking specifically about potential downsides.

3) Is a 305 on an 11.5" or 12" wrong? I would never say that. There is no right or wrong. There are only things to consider.

4) Cosmetic appeal never entered my mind. My opinion on the cosmetics is that all of these tires are low profile and sporty looking. I don't consider the cosmetic aspect much more than that.

By the way, I was looking at my tire specs from the standpoint of the stock setup I started with and the one I ended up with. The stock 305 RE050As have a section width of 12.6" on the stock 11" rim. My new 325 PSSs have a section width of 13.0" on the 11.5" rim. Comparing the two, the rim grew by 0.5", while the section width grew by 0.4". Less bloat? Please tell me my tires aren't bloated!

So, I will point out (what you already know) that your bloatometer relies on the assumption of the same tire construction. I switched brands.

Denis, you should know by now how I react to statements like "The GT3 RS, the car Porsche developed to be the ultimate track toy, comes stock with a 305 on a 12". Think this says it all."

Back and forth is how debates go, and debates can be one of the best ways of enlightening ourselves. Like, for example, a half inch is 12.7 mm, not 25.4. My friends and I debated all the way through grad school--to the point of yelling at times. But we always retired to the local bar afterwards with no hurt feelings and usually wiser.

Louis

dianic

Louis, I should mention that Denis used to be involved in the making of rims. He does have an opinion, and an educated one at that matter.

The 305mm section and the 325mm section refer to the width from the outside of the rim, up the profile, over the tread of the tire, and down the profile on the inside down to the rim.
The 30 in 305/30 refers to the % of that width which is divided in the sidewalls. So, its best to use a calculator like the one Denis linked to, to make sure that you are taking all the variables into play. And that % figure really screws things up most of the time if you dont carefully think about it.

:-)

L_perm

You're right Nick. I have been talking in simple terms--assuming a constant sidewall height, to be specific. That's at least part of the reason why the section width doesn't increase as much as the rim width when moving from an 11" rim with 305/30s to an 11.5" rim with 325/30s. An extra 20mm section (325 v. 305) with a 12.7mm increase in rim width (11.5" v. 11") would seem to indicate an increase in section width of 7.3mm. But, the sidewall absorbs some of the 20mm.

In my case, I also switched from Bridgestones to Michelins, which likely also affected the change in section width going from 305 to 325.

Hopefully Bob is getting something out of all of this.

Louis

strato58

I am getting something out of this...you guys are a wealth of knowledge. I appreciate all the input. At this point I dont see anything wrong with an 11.5 rim with a 305.....and Ill have half the "bloat" that id have with an 11" rim....
thanks all so much.

kosmo

Louis, any issues tripping the cars ECU by going to a taller (325mm) tires?

L_perm

Don't know yet. The wheels/tires are stacked up in my garage, and the car is up on jack stands awaiting upgraded turbos back from Germany.

I don't expect the taller tires to matter, except for the speedo. What matters is the diameter differential between the front and rear. The stock tires have a small differential in size, and my combination has a slightly larger differential. Obviously, the PTM has to allow for some differential in wheel speed for normal variances in tire size, pressure, etc. Others have reported no problems with my combo, but I won't know for sure until I slap them on and go for a ride.

Does anyone know the percent difference in wheel speed between front and rear at which the PTM kicks in?

Louis

TT-911

Your front and rear combo is 100% fine and within spec.

L_perm

Good to hear! Thanks Denis.