"02996ttx50

thanks very much. i'm going to take that as a definitive answer, esp since you've pointed out those OEM b'stone numbers and that i'm within at @ 12 revs per mile differential. again, thanks.

adam_

The revs per mile is more precise, in terms of a published number. The published spec is in 0.x or one tenth. I would not be surprised if the PS2 in OE sizes (225/40; 295/30 is within one tenth- yet has a 3 revs per mile 'mismatch')

To comment on a few posts above- the proposed sizes 225/40 with 315/30 has a 12 rev per mile mismatch- FOUR TIMES the heat and impact on the differential.

(Common issue- people want to upgrade sizes, but don't want to have to replace the fronts when there is so much tread left... )

This is the root of the issue of using outer diameters- you get stuck on %s, and lose sight of the issue which is the revolutions the diff must turn to compensate.


A

c32AMG-DTM

The OE Bridgestones, in the OE 225/40 and 295/30 fitments, are 14 revs per mile different.

Said differently, the PS2s in 225 and 315, apparently are still a closer match than the OE Bridgestones in 225 and 295. So less heat and impact on the front diff than an acceptable OE solution.

Honestly though, if someone wants to run the 315 rear, they really should upgrade to the 235s in front (i.e. go to the full 996 GT2 fitment - not just the rears). JMHO.

SSST

+1. That's what I'm doing when it's time for new tires.

"02996ttx50

my dilemma, absolutely.

i find myself obsessing about this, but i attribute that to my car having thrown the abs/psm CEL after switching the sizes! so i will probably be switching out the fronts sooner than i'd have liked or necessarily "needed". if only i would settle for the hankooks again this'd be a non issue! thanks for the input also.

"02996ttx50

no doubt you're right. i'm already lowered gt2 height and with the mo30 suspension so i probably(?!) should get the 235's. i was just trying to squeeze the life outa the fronts. just hopefully not at the expense of unnecessary wear to my front drive. i got ps2 315/30/18's @ $330 each. what was i gonna do!?

Brenton

SO the bigger question????
I am not worried about $$$$$

Is PS2 235/40/18 315/30/18
the tyre of choice?

Dose this dial a bit of understeer out of the car

"02996ttx50

the gt2 fitment would be my sizes of choice. i've run sets of pirelli's/hankooks and ps2's on two different tt's, and the ps2's are my preference. ymmv.

White Trash With Cash

I dont think it is. I suppose it is an easier relationship to understand. Rotations per mile is just a calculation derived from the diameter.
I dont think the tirerack has actually mounted these tires and had someone roll them down the street counting how many times it rotates over a mile..

c32AMG-DTM

Agree, I don't think TireRack has... but AFAIK it's a manufacturer's spec... so, presumably, they (the manufacturers) figure out the revs/mile calc for the tires they produce.

Brenton

Has anyone found if they are going to make PS3's in these sizes?

If not PS2's it is

White Trash With Cash

Ok, so if it is calculated, the only thing you need to find the rotations per mile is the diameter of the tire.
rotations per mile =1 mile(5,280ft) / circumference of tire (2(pi)r)
But, again you need the true (measured) diameter.

adam_

haha. I did too, then just bit the bullet- I have a set of 225/40-18 PS2s with 50% tread rotting in the barn.

Yes, tire size of choice. But when I bought HREs I sized the front rim to 9"...so you can run a 265/35-18 up front matched to a 315/30-18 on the rear. Sweet. I can actually also run a 235/40 on that 9" rim, so it seemed like a no-brainer. Surprised not more talk of this- common on track cars from what ive seen.

Going to 235+315 really doesn't change the 'balance' so no big change in understeer, IMO. Y

Agreed

You actually need the 'effective diameter of the tire when loaded' (this might be what you are calling the "true" diameter). Which makes me ask, "is the rev per mile an actual value and at what load is it measured"? At rated load??

Anyway....