naschmitz

I am shopping for race compound tires for my 2006 C6. The tires will be mounted on a set of stock wheels. I am considering the Hoosier A6s in the following sizes:

Rear: P295/30/19 26.1" diameter
Front: P245/40/18 25.7" diameter

I am concerned that the difference in diameter is too small as I have read second hand accounts on the forum that 0.4" is not enough. I can reprogram the rear tire size in the ECM using HP Tuners, but I didn't see any settings for the ratio. Or does turning off traction control and active handling make this a non-issue?

As an alternative, I could use this combination:

Rear: P295/30/19 26.1" diameter
Front: P245/35/18 24.7" diameter

This would put the difference front-to-back at 1.4". Would that be better? I am thinking that this setup would LOOK better on the car as the aspect ratio of the tires is closer to stock.

For reference, the stock tire sizes are below.

18x8.5 wheel 245/40ZR18 25.7"
19x10 wheel 285/35ZR19 26.9" 1.2" difference in diameter

And yes, I know that I probably should ditch the stock wheels and buy 17"s all around, but that needs to wait until next season.

As you can see I have lots of questions so your inputs on some or all of this would really help.

Thanks, Bert

SteveC6R

The A6 245F and 295R is the best choice for AutoX on stock wheels. I used that setup before I went with bigger wheels. It worked pretty well.

G.J. Dixon

I'm debating about trying Hoosiers 275/35 R18 for the front and the 295/35 R19 for the rear.

If you plan to run SCCA Super Stock class, then you are stuck with stock wheels, other wise run whatever wheel/tire combo you want.

It would certainly be cheaper to use 18's in the rear, which would also make a lot more tire sizes available.

I would be turning off the traction/stability control anyway, so tire diameter shouldn't matter.

SteveC6R

That combo gives you too much rake, IMO.

G.J. Dixon

How does that give you too much rake?

245/40 R18 tire diameter is 25.71

275/35 R18 tire diameter is 25.57

A difference of 0.14 tire diameter is going to make that much of a performance difference by giving the car too much rake?

EvilBoffin

G.J. -

First- I assume you're referring to the 295/30 Hoosier A6, not a 295/35, cause I don't think they offer a 295 in that aspect ratio.

Based on the A6 specs on TireRack.com, you should be fine with those sizes, even if you were to run with Active Handling turned on.

Check this link :

Hoosier A6 specs on TireRack.com

The 275/35 is 25.2 inches tall, and the 295/30 is 26.1", so the F-to-R difference in diameter is .9".

The stock tires are 25.7 & 26.9, with a F-to-R difference of 1.2".

You'll probably reduce understeer and pick up some additional front grip with the 275's over the 245's, assuming that's what your car (and your driving style) needs. And theoretically, since the 275/35 is also a lower aspect ratio, it shouldn't hurt the steering response/turn-in, but that remains to be seen in real life...

EvilBoffin

Also worth noting -

The OP mentioned changing the tire sizes in the computer using HP Tuners.... Well, that is NOT legal in SCCA Stock classes, nor is the tuning of any other CPU parameters with software or hardware tuning devices.

Not trying to get preachy - just be aware that your competitors may take offense if you start 'tuning' your STOCK car in the pits...

naschmitz

I like the idea of running the 275s on the front, but did not consider them due to the stock 8.5" width. The public Hoosier data says you need a minimum of 9.5" rim width for that tire -- but does it work anyways?

Congrats on winning your C6 by the way!

Good to know. For SCCA classification I think my Elite Engineering heel-toe gas pedal bumps me out of stock too. In my case, the stock wheel size limitation is a budget constraint, not an intent to comply with SCCA classes.

I run events hosted by several Ford clubs (SCMC, SVTOA, and NESHOC) that have only rough car classification systems. There are several C4s, C5s and one C6 Z06 and all run these events on R-compound tires. Due to the special wheel sizes of the stock C6 I was interested in everyone's input here, so thanks everyone.

I am also considering the Toyo R888s in 255/35/18 and 295/30/19. These are not as quick as the Hoosiers, but I could use them for wet events as well.

G.J. Dixon

Oh yeah, I forgot the rear profile size was 30. Wouldn't that actually reduce rake then? I thought Corvettes have some rake built in?

EvilBoffin

G.J. -

With a F-to-R diameter ratio SO close to stock, I seriously doubt this could cause any noticeable problem with the rake.

From the research I have done here, you should be fine with any F-to-R difference of .6 inch or more...

As an example - Michelin PS2's come in a 275/35/18 & 305/30/19 size combo that many C6'ers have used successfully, and they are right on that .6" ratio limit.

BTW - there are guys running 18x10.5 wheels with 315's on all FOUR corners for auto-x, without any issues.

G.J. Dixon

I figured that for the speeds achieved during autocross that any small change in rake wouldn't matter.

I'm just trying to stuff the widest tires on the wheels to maximize front grip, while still maintaining a decent contact patch.

I know it would be a lot more important for the HPDE's and TT's where the speeds are much higher and aero comes into play.

G.J. Dixon

Thanks

It seems like you are about 1/2 hr from Fort Devens, Mass.
Boston BMWCCA and NER SCCA run there all the time. The SCCA has a National Tour there in June.

Once I get things orgainized with my car I'll be travelling up there a lot, unless I'm driving someone elses car.

I'm only suggesting running stock class, with the limited amount of modifications you seem to have on your car, they will put you in a class that the car will not be competitive in. By running stock, at least things should be a little more fair.

TedDBere

I would never let change in rake be an issue when choosing tires for a competition Corvette...you can easily set the rake to whatever you want with these cars, so pick the tires with the largest contact patch and lowest profile that you can get on the wheels and adjust the rake afterwards. IMHO


BTW GJ, I have those tires ready for you to try when you want them. Let me know.

G.J. Dixon

Agreed.

Cool! I'll be at the MSNE Test and Tune on March 9th. If you are planning to go to that, bring them along.

TedDBere

I'll see if I can make it.

And BTW, I don't know about the importance of rake in a C6, but it's really important to get it right in a C5 in order to get the car to rotate in Autox.

DarthStimpy

Are you leaving me and bumping up to the big boy class?

naschmitz

Hi Mark,

I saw your posts about using C5 Z06 wheels and I think that is a good move. Last summer I got a set of C6 wheels, sensors, and new GY runflats for $1035 so I have a second set of stock wheels I can use for mounting dedicated auto-x tires.

I just couldn't find anything that really excited me with a wear rating >150 to stay in street tire class that fit the stock wheels. But there do look to be three decent options <150 wear rating with the Hoosier A6, R6, and the Toyo R888s.

Bert

DarthStimpy

yeah, that was my conclusion as well, thats why i went with C5z wheels and kumho MX's.

good luck in ER, it's going to be very competitive this year.

C6 4 AL

The R888's are going on my C6 on Monday, with a event scheduled on Sunday. I will let you know how they do. I have been running the SK1 class but unable to find a good 19" rear for now so I am switching to SS at this time.

naschmitz

Good luck Sunday, Al. I would be really interested in what you think about the R888s. 41 more days until our season starts here.

Bert