J95SNAKE

My autocross season ended today and I will be looking for a new set of RS3's for next year. I was told that a 245 on my 8" wheel would handle better than my 255s. I'm running STX which allows for a 9" wheel but I already have the 8's and I do not want to increase costs. Anyone use this philosophy on going to a smaller width tire when you can use wider one?

Norm Peterson

Mounted on the same width wheels, a 245-width tire will perhaps handle a little more "crisply" than a 255-wide tire. But that doesn't say much about things like ultimate grip or overall gearing. So you'd have to test both sizes on the same day on the same course (or courses) to determine if the desired end result of lower run times with the 245 tires would actually happen.

Just as a general comment, the S197 is awfully heavy for STX, and as a result needs to be running on the best and widest rubber that's legal on the widest legal rims to have much chance against the lighter competition (that you can expect will be running on the widest allowable tires and wheels anyway).

The financial side may not be as clear-cut as you seem to think (aside from the possibility of it being completely unreasonable or irresponsible financially to upgrade to STX wheel and tire limits). If you run the numbers, you may find that keeping dedicated competition tires separate from your dedicated street tires may break even within a couple of seasons, assuming that this car is your daily-driver. Not to mention the expense of having to replace your street tires at tread depths still entirely useful for your auto-X use.

Note too, that street-driving your "competition tires" will heat-cycle some of the ultimate grip out of them and cost you time at events that you were counting on the competition tires to give you in full meassure.


Norm

J95SNAKE

These tires are my dedicated tires for autocross. I have another set for daily driving.

I have been told that this car is very heavy for STX and that it wouldn't be competitive. Cost is what drove me to the class and we (co-driver) finished higher in the PAX this year than last year with R comps and less mods. My co almost won the overall PAX for the year this year. We have a small club so this class isn't killing us as far as being competitive.

I was told that a 245 would handle better because it fits the wheel better than a 255 does. I agree hat it fits better but weight is was pushed me towards the 255. I'm allowed a 265 but that is too wide for the wheel. We already had the wheels, so we stuck with them and used a 255 because of cost. Money is the driving factor, especially since I have to retire next year. My pay will be cut in half so I need to manage my income until I get my sugar momma plan in action.

Norm Peterson

Give some thought to how close to running on the rev limiter you are on your normal lots and course designs as it is. A 245/xx will be revving just a little higher if you can still achieve the same peak "straightaway" speeds.

I'd still look really, really hard for some used 9" wide wheels. Car shows/swap meets/at the drag strip if you don't want to do the ebay/Craigslist thing. Maybe on closeout at an established company via online.

Even if they're heavier than necessary, the inch wider will buy you better results than the weight would ever cost. If it matters, you'd still be able to run 245-wide tires on 9" wide rims . . .


Norm

J95SNAKE

Thanks for the info Norm. I'll look around and see what I can find.

UrS4

How does a 255/40/17 fit on an 8" rim?

The approved rim width for that size tire is 8.5-10 with ideal rim width of 9". It's diameter is 25", which is over 2" shorter than stock.

245/40/17 rim range is 8-9.5 with ideal 8.5", with a diameter of 24.7.

Stock diameter is 27.2"

I guess that small size tire really helps with gear but I don't see how you are maximizing tire grip putting them on such narrow rims.

Norm Peterson

It is very specifically and very narrowly intended for SCCA "Street Touring eXtreme" autocross competition, which is ABSOLUTELY NOT an endorsement for general use. Use at autocross entails various additional risks over and above what are normally anticipated in normal street driving, which the entrant understands and freely acknowledges.

Since autocross runs are of such short duration - generally under 60 seconds, and separated by significant time stopped (at least 5 minutes minimum time between successive runs by the same car) that the "minimum rim width" specs you see in the various Tire dimension charts do not really apply.

Those tables are intended for tires in long-term general service, where fatigue effects due to extended time being driven at temperature are much more critical. Extended time being the operative term. (PS: if you think that 265-wide tires on 8" rims are bad . . . you really don't want to know what tire and wheel size combinations that folks in the "F-Stock" class typically run.)

The life of a dedicated set of STX tires is unlikely to ever exceed about a hundred miles. Either one or more tires will be corded by then, or the grip level of the tread compound itself for the whole set will have been heat-cycled down into utter non-competitiveness.


Norm

J95SNAKE

What Norm said. We have guys in FS that run a 275 on an 8". They cheek out bad too but they work. I wouldn't do it though.

Believe it or not 255s came stock on my 95 cobra and it had 8" wheels. These wore fine and didn't cheek out like guys running a 275 but 255 is as far as I will go. RS3s don't come in a 265 so I'll probably stick with the 255. The 245 that is available is shorter and I'll probably hit the limiter more often as I already come close to it now.