RTR Tire Size

[SilverMustangGT]

I just purchase a set of light weight 18" wheels and now I need to purchase tires. Has anyone tried autocrossing on 275/35/18 street tires installed on the 18x8.5" rim? I'm looking at the g-Force Rival, Dunlop Direzza II, and Hankook R-S3.

I know that drivers have been successfully using the Hoosier A6 size 295/30/18 on 8.5" rims, but those tires are designed just for racing. I read an article somewhere that was questioning the need to use a wider tire when competing on tires that aren't r-compound...tires like the R-S3 and StarSpec

I have been competing on the Hankook R-S3 size 255/40/17 the last couple of years, so I think it's safe to use 265/35/18 on an 8.5" wheel.

Rob

 

[JesseW.]

1. try it
2. see if it explodes.
3. report findings back here.





really, its not great, but i don't see why it won't work. its probably going to sacrifice some sharpness to turn in, the wheels will wiggle back and forth in the sidewalls more.

 

[zquez]

Have you looked at the manufacturers recommendation on what tire size will fit what wheel? I'd use that to make a decision.

 

[Paladin LL 06]

Tirerack.com has a chart. When I bought mine it said 9.5" wheel for 275's. But I don't know anything about racing tires.

 

[Norm Peterson]

Have you looked at the manufacturers recommendation on what tire size will fit what wheel? I'd use that to make a decision.

In SCCA's Stock category autocross (and anybody else's autocross series that follows SCCA's rule sets) you will see some tire and wheel size combinations that have no business at all being run on the street.

It's the "unintended consequence" of setting very restrictive wheel rules (stock width) but allowing wide-open tire choice (I think being "DOT legal" is the only tire restriction - no real race tires allowed). It's the extremely short durations of autocross runs combined with significant idle time in between that lets people "get away" with some combinations that would be really poor for anything else. Typically too, the competitive life of R-compound tires and even the 140 minimum-treadwear "Street Touring" tires is a hundred miles or less (before they heat-cycle out or cord).

Basically, if you're going to run SOME CRAZY "SCCA F-STOCK TIRE SIZE AND WHEEL WIDTH COMBINATION", you owe it to yourself and others around you to ONLY USE THEM AT AUTOCROSS.

FWIW, 275/35's are generally accepted by the mfrs (Tire & Rim Association, actually) down to 9" wide wheels. 9.5" is measuring width, 11" is max recommended.


Norm

 

[SilverMustangGT]

In SCCA's Stock category autocross (and anybody else's autocross series that follows SCCA's rule sets) you will see some tire and wheel size combinations that have no business at all being run on the street.

It's the "unintended consequence" of setting very restrictive wheel rules (stock width) but allowing wide-open tire choice (I think being "DOT legal" is the only tire restriction - no real race tires allowed). It's the extremely short durations of autocross runs combined with significant idle time in between that lets people "get away" with some combinations that would be really poor for anything else. Typically too, the competitive life of R-compound tires and even the 140 minimum-treadwear "Street Touring" tires is a hundred miles or less (before they heat-cycle out or cord).

Basically, if you're going to run SOME CRAZY "SCCA F-STOCK TIRE SIZE AND WHEEL WIDTH COMBINATION", you owe it to yourself and others around you to ONLY USE THEM AT AUTOCROSS.

FWIW, 275/35's are generally accepted by the mfrs (Tire & Rim Association, actually) down to 9" wide wheels. 9.5" is measuring width, 11" is max recommended.


Norm

Norm, I agree with your statment and these tires will be mounted on a set of wheels dedicated for Autocross. For local events I normally install the wheels the day before the autocross and drive to the event with the the wheels installed. For anything out of town, I just install the wheels on site.

A lot of you are probably asking why don't I just buy Hoosiers since I have a dedicated set of wheels and don't really drive them on the street. We don't have any cars that compete in F-Stock in the local regions, but we normally have about 10-15 cars that run in RTR. Also, I was able to get two years out of my last set of tires.

Rob

 

[Sky Render]

Yeah, I wouldn't stick 275s on an 8.5" wheel...

 

[cito]

There are no clear answers with these sorts of questions. You can get away with wider A6's because Hoosier designed them that way. We ran 285 A6's on 8 inch rims on my RX8. With street tires, my understanding is that you do not gain that much from having wider tires on narrower wheels because the sidewall dynamics and the quirky contact patches that can occur. What I don't know is how forgiving the specific tires you are looking at are with respect to rim width. You also have to consider what sizes work for your car and pick the best fit.

If I had just two choices, I would run a 275 on my 8.5 inch rim before I would run 245's in the same tire. Frankly, I would call Tire Rack and ask for Woody, he's an autocrosser, and see what he recommends. I think Chris Harvey is also at TireRack, but I can't remember what his sales name is.

 

[SilverMustangGT]

There are no clear answers with these sorts of questions. You can get away with wider A6's because Hoosier designed them that way. We ran 285 A6's on 8 inch rims on my RX8. With street tires, my understanding is that you do not gain that much from having wider tires on narrower wheels because the sidewall dynamics and the quirky contact patches that can occur. What I don't know is how forgiving the specific tires you are looking at are with respect to rim width. You also have to consider what sizes work for your car and pick the best fit.

If I had just two choices, I would run a 275 on my 8.5 inch rim before I would run 245's in the same tire. Frankly, I would call Tire Rack and ask for Woody, he's an autocrosser, and see what he recommends. I think Chris Harvey is also at TireRack, but I can't remember what his sales name is.

Thanks for the suggestion. The wheels I bought came with Hoosier A6 tires installed, so I will just probably stick with them until the end of April. I have always raced the Mustang on street tires, so I'm curious if the Hoosiers will improve my PAX time.

I'm leaning toward the Dunlop Direzza ZII in either 265 or 275.

Rob

 

[lost won]

RTR tires for Mustang

Hi, Rob

Interesting thread. I run a 2012 GT with Brembo in FS. I'm running 19x9 wheels and Hoosier A6s in size 295/30-19. I've often wondered if running RTR-FS with the better PAX index(.8134 for RTR, vs .830 for FS) would work for a better overall PAX score. R comps vs "street tires".

Theory is, the two approaches should yield the same PAX score, right?

Then there are all those pesky variables.

Such as: Driver skills, and car suspension prep.

Hoosiers, especially fresh ones, stick like glue. This has an upside in that a mediocre driver can often do way better than his skill level because the Hoosiers allow more reserve grip to cover up all manner of driver error.
With street tires, the driver HAS to be much more smooth and careful.
In the case of a relatively heavy, softly sprung, camber challenged, and powerful car like a new Mustang GT, driving fast on street tires can be fairly dicey. Driving on Hoosiers, at the limits, can be great, right up to the point when you suddenly realize you're looking over your shoulder to see where you're going...

In either case, we're stuck with F stock car prep rules. Basically, you can't modify springs. Shocks help, but not enough. Camber for the front is very limited. Power is the only trump card, but to play it off the corners, you gotta have grip. Hoosiers have it (so far), and street tires don't handle 380 ft/lbs of torque well enough in a rear wheel drive, stick axle car.

My calculations, (using my own experience in about 15 local events), seem to show for most people, the car/driver will pax out better with the FS/Hoosier combo. Yes, I know the Hohos will cost more(2X) on a dollar per lap basis, but you probably will go faster, and pax better.

I'm still watching the newest street tires, though...

John

 

[boardkat]

another 5.0/3.31/brembo f-stocker here. running 275/35R19 ZIIs for my local events this season, and saving my sticker A6s for tours/pros/nats. no denying the relative longevity of street tires!!

just ran my season opening local event last weekend; was surprised how well they did out of the box (low-mid 50deg that morning, single driver). while their ultimate grip is nowhere near a ho-ho, they can take more slip angle than you'd think and still set. as john postulates above, you must be very analog with your right foot, especially on corner exit. but if you manage to get things to click, you may surprise yourself. i pax'd 1st overall by a few tenths on a low-mid 40 second course against some decent talent:

this was only my 2nd event ever in the car (and first on street tires) - although the 1st was @ an evo challenge/tnt weekend where i was running on A6s with mr. sam strano and a few other hotshoes. so i moved up the learning curve substantially that weekend!! in any case, i'm firmly in the camp that believes the car can do well if you run in a region with a street tire multiplier and drive well.

for an 18x8.5, i think a 275 street tire might be a little much, but your only other realistic option seems to be a 245/40R18, so it might be worth trying - i can attest that that the ZII has a *very* stiff sidewall, so it "could" work. hoosier, if you ever decide to go that way? not a problem - i ran a 295/30R18 A6 on my GXP 18x8's a few years ago; they mount and run just fine.

 

[SilverMustangGT]

I have also wondered which class would be better for pax. I have driven on Hankook RS3 tires the last couple of years in a 2008 Mustang GT using factory 17x8 rims. I used 255/40-17 tires. After prepping the car last summer for F-stock (shocks, sway bar, slotted struts, alignment) the car became competitive in RTR and pax. Two top 3's in pax, a few top 5's, and several top 10's. This was using .98 as the multiplier instead of .975.

Even though i was competive, I also think the Hoosiers are the way go. I think the ability to use wider Hoosier tires will be the difference...295/30/18 versus 255/40/17 or 265/40/18.

I will be driving the first few events on Hoosiers, so I hope to have a decent comparison. The tire ages are very similiar. The Hoosiers have a few heat cycles and the Hankooks were more than a year old.

Rob

 

[lost won]

Wow! Nice results, Dennis! Please keep us in the loop as the year progresses.

Looking forward to seeing the results of your adventures with Hoosiers, too, Rob.

I'm going to stay with Hoosiers in F Stock, for now.
Squeeked out another win in San Diego March 3.
Ran the EVO Phase I in El Toro today. Definitely picked up some ideas for improving my driving!

Good luck and best regards, everyone.

John

 

[SilverMustangGT]

Good to see the Mustangs winning.

I feel like the handling on the Mustang is very good, but trying to put the power down when exiting the corner has been the biggest challenge. All of our events are on sealed asphalt that's very slippery. And it doesn't help driving on two year old tires.

Can't wait to try the Hoosiers.

Rob