Need some help with track day tire sizes--987

[MikenOH]

've got an 06 Boxster with an extra set of 18" rims and standard PS2's that I use for DE's. These tires have worked well for the purpose of having a tire that you can drive to the event on and use at the track. They've worn well and, while not as sticky as R compounds, can take the heat, handle predictably and have a sufficient amount of dry traction to have fun.

These tires are about done and I have a chance to get a set of Extreme Performance tires for essentially nothing, but can't get a rear tire in my stock size--265/40/18. The closest I can get is a 275/35/18 which is about .6" less in OD vs. stock.

My question is: will this smaller OD tire create problems in handling at track events? I like the idea of a free tire set but not at the expense of creating problems at high speed events. If this were for auto-x, I wouldn't think twice about going with the smaller tire, but..

I plan to keep the 235/40/18 size on the front; the replacement tire has performance similar to the Dunlop Z-1.

Thanks for any help.

[Topless]

Good question. A smaller diameter on the rear should not affect basic handling and balance but may confuse the computer controlled ABS and PSM. The rule with a 986 is to keep diameter within .3" difference front to rear. I am not sure if it is the same on a 987. Does your owners manual address this?

[MikenOH]

on the 987, the front -rear sizes must be within a +5%-5% window or the ABS acts up. So if your front has an OD of 25" your rear can't be any larger than 26.25".

In my case the tire maker has a 245-35/18 tires which will work well with the 275-35/18 on the rear. Both OD's will be about .6" smaller than stock but will be in the 5% range and the F/R OD differential will be properly maintained.

Nothing like a set of free track tires, especially if they fit..