NTMD8R

2008 Z06, 515 rwhp, serious autocross

I am thinking of going with Toyo R888R tires.
I know, for serious autocross, use Hoosier A7s... I have them now.
But I want something for damp days, and for street, but still competitive.
I used to run Toyo R888 tires, and they were perfect for this.

So I must now get Toyo R888R tires... great.
But I was thinking for going really wide up front... 315/30/18,
and 345/30/19 rear.

Sam Strano runs these sizes on his stock wheels, and he says they fit/work great.

Stock front wheel on the Z06 is 9.5" wide.

I would just like some more opinions before I do my leap of faith.

Any tire/rim experts out there who can tell me what might/can/will happen if I do this ?

Thanks

stevebz06

My initial reaction was that they would be too wide and the tread would bow out giving you a narrower contact patch, but if Sam is using them, I guess they would work. I don't think he would lie, but he is a racer.

NTMD8R

Sam is using A7s in these sizes... I should have mentioned that.
Race tires have a stiffer sidewall, so they can handle the added squeeze with no issues.
Street tires will undoubtedly cup with that wide a tires on the 9.5 rim.

I have found this out by doing some extensive research.

So I COULD run Toyo R888R tires with the widths I mentioned on the stock wheels.

But... just to play it safe, I think I'll narrow down my width a bit (this is for the fronts).

stevebz06

The A7 is designed with a "cantilevered" sidewall so that you can get a wider than normal tire on a given wheel. It probably adds a little weight but pays back with a large contact patch.

I think it's probably a smart move to not try to put the 315's on the front wheels. If they are too wide you would actually have less traction than the properly sized tire, and you run the risk of popping them off the bead which is a safety issue.

NTMD8R

Thanks Steve,
Yeah I'll go with 295s on the front.

0Luke@tirerack

here's an interesting read regarding your question ...

Link

stevebz06

Thanks Luke, that shows how valuable testing is, and that brings me to another question: what are you recommending for DOT 200 treadwear Z06 sized tires for SCCA autocrossers? I have a friend who is ready to sell her car because she can't find anyhthing.

cpt11gs

I'm running the BFG Rival S in 275 front 335 rear, 18" on my C6 Grand Sport, and they work very well. For a C5 Z06, the local folks here run the 275 fronts with 315 rear Rival S.

stevebz06

I should have added that her car is a C7 Z06. It seems that the choices narrow down a lot when 19 and 20 inch wheels are involved.

cpt11gs

For the fronts, there are some local Camaro 1LE folks running the RE-71R 305/30-19 on a 10" wide front wheel, so that's an option. The 20" rears are a big issue though. If you could drop the rears down to 18", you can run the 335 BFG.

stevebz06

She seems to think that there would be some computer issues running smaller than stock wheels. But running different makes of tires front and rear is hardly an ideal situation. Even if it works it means there would be no contingency awards available.

I personally have put on hold any idea of buying a new car that runs these big diameter wheels at least until the tire suppliers catch up.

The current rules allow going up or down only 1 inch in diameter.

cpt11gs

Then you can go down to 18" front, 19" rear, and run the 275/35, 305/30 RE-71Rs. You really want a bit more tire in the rear, but that's as wide as the RE-71Rs come in, and BFG only goes to 18" diameter.

Computer won't know or car about wheel diameter, only the difference in overall diameter, and those tires should be fine. I'm further away from stock than those are. Course who knows about the C7 computer, but my C6 does not care.

NTMD8R

I can chime in a bit here about the C7. I had a 2014 Z51 M7 for 2.5 years.
I had to go with 18/19 wheels and used Re71 tires.
With the correct alignment (rear caster !!!!!!!), the car COULD handle great.
I say "COULD", because I never could get my car to the correct alignment.
But a friend has a 2015 A8 with the same wheels/tires... his handles unbelievably well.
And the computer doesn't care about the diameter (wheel or tire),
it cares about the difference between front and rear.
As long as you are 1" to 2" different, my experience says that is just fine.
I have run from stock (.7" diff), to 1.7" diff, with no issues.

As for the C7 Z, you should be able to run smaller diameter wheels, with the RE71 tires
mentioned above. The computer won't care unless your front/rear diff is way off.