mdrums

Trying to decide on some new tires to DE on.
2009 Porsche 997S
I currently run RA1 or R888 245-40-18 and 305-35-18 with CCW C10 9x18 and 12x18 wheels

I was looking to try and use 255-35-18 and 315-30-18. Here is the problem

The 315-35-18 sized tire Toyo says max rim width is 11.5" and load rated to 98Y 1653lbs.

The 305-35-18 sized tire Toyo say 12" rim max and load rated to 100Y 1819lbs

My stock PS2 rear tires are load rated to 101Y 1714lbs.

So you can see the 305-35-18 is better than spec but the 315-35-18 is lower than spec and only is suppose to fit up to 11.5" rim

Thoughts and opinions??????

SundayDriver

Why do you care about the load rating? All are well above the weight on that corner.

utkinpol

why do you think you cannot use 335 tires which are intended to be used with 12" rims?

Scootin159

Even with the inferior 1653# load rating, the 315's would hold more weight than your rear suspension could sustain. Unless you're filling your rear seat with bags of concrete for the DE, you'll be fine. Even then you'll probably still run out of space in the back before you exceed the load capacity of those tires.

Without question I'd go with the 315's, or the proposed 335's (if they exist, and would clear the fenders).

mdrums

I am useing the load rating of my stock tires as a guideline....not really sure what else to use as I would think Porsche has this figured out through their testing. Thought and opinions?

mdrums

335 tires in the rear are not needed I would think. I do not think I could get a properly sized front to match up with the 335 rear. If I use 335 rears and 245 fronts I would think that would cause less rear rotation and more front push.

I really wish I knew the real answer but so far only a very few of us here track a 997.

SundayDriver

I think that load rating is a by-product of the design, not a major design target for passenger cars. If you are really concerned about Porsche having it exactly right, then only use the OE tires with the Porsche designations. But even then, realize that the mfg has their own set of criteria as to what is important and that may differ from what you value. Personally I have never even looked at load rating unless the tire was for a truck or trailer.

mdrums

Yeah, I am just learning and trying not to make a mistake. I do not know who to believe.

I've been using 245/40 and 305/35 and wanted to try out the smaller diameter of the 255/35 and 315/30.....but....the load rating on the 315's is lower and the recomend max rim width is 11.5" and I am at 12"

So far I've received advice on the load rating but one one has said anything about the rim width issue. Anyone know if this will be safe?

SundayDriver

Hard to give a solid answer about rim width, but here is my opinion/experience. I have gone outside the suggested widths with no problem. BUT, you never really know - not all tires are built the same.

e.g. - There are tires that are built for wider rims and there are cantilever tires (slicks for certain classes) that are designed to run on very narrow rims for the width of the tire.

It all comes down to whether the tread is flat in the application. I would not hesitate to go 1/2 inch outside the suggested rim, but would check it with a pyrometer to ensure you are getting a decent footprint.

utkinpol

some folks at auto-x do kinda crazy things, like putting 275+ tires on a 8" wide rims. there is a common conception there apparently that 'the wider is the better' no matter what, sort of. kinda difficult to understand that but i would guess slow speed cornering is probably different in mechanics related to tires than DE fast speed cornering requires...

i would love to see a generic answer from purely mathematical standpoint - how do engineers 'compute' an ideal tires width and setup for a particular car and how it applies to 997 car in particular.

beentherebaby

The DOT tire load rating is the maximum sustained load for a tire used on the highway. It really is meaningless unless you are using undersized tires for the application and running your vehicle at it's maximum rated weight, 24/7.

mdrums

Thanks for the explaination.

I'm still trying to decide on either staying with

245/40/18
305/35/18

or going with smaller diameter but slightly wider

255/35/18
315/30/18

cello

Cant help, but following along with interest. What is your goal here? Do you need the extra width to alleviate a problem? The 245/305 combo is mainly considered "what to run" on 997.2s up here. Have not seen the larger tires in the field and, more importantly, have not heard a reason why they would be necessary. What do you know? Have experienced? This info might also draw out additional responses. SundayDriver is on it - will it give you an 'effective' (read: greater speed and/or handling) increase in contact patch? I think that the main question here (but you might be after, remedying, something totally different and particular to your car and track)... Interested & thx for the posts.

85Gold

Mike,

Something is being lost in the translation you stated ""The 315-35-18 sized tire Toyo says max rim width is 11.5" and load rated to 98Y 1653lbs""


http://www.tiresdirect.net/TOYO_Prox..._P1527C200.cfm

Minimum rim width is 11.5" for the 335/30 18

Peter

kush07

Am I wrong, or is there like a 1-2mm difference in diameter?