How wide can/should I go?

I've yet to do any research, but i thought i'd throw up a thread first. My original plans were to run 18x12's in the rear with 315/30zr18's and 18x10 in the front with 275/30zr18's. My local forum is full of "haters" and I basically got told it was a terrible idea due to the rears only wanting to go straight, therefor causing terrible understeer. I'd like a second, educated opinion on this.

I was looking at Hoosier A6 autocross slicks, but as per usual, local forum members didn't hesitate to shoot me down and name 100 reasons i shouldn't be doing this project altogether. Has anyone ran something this wide in the rear? Positive/Negative impacts?

Another point they made was that running 18x12's would be extremely heavy, and that 17x10 would be the biggest they'd go. this IS an option, with 175/40zr17's or similar all four corners.

Please chime in with your opinions/suggestions, between 17/18" rims, staggered widths, etc. Thanks! P.S. i will continue to research, but wouldn't mind some suggestions either.

last track day i had 275/40/17 front, 245/45/17 rear and it worked out really well with the v6.
with a v8 i was gonna go for 275 all around, but came across a good looking set of 4 c5z06 rears (18x10.5) with staggered tires, so i went for it.
never ran with them, but woulda been pretty fun to try out

18x12 is definably a lot of tire.

Yeah it's lots, but TOO much? I'm leaning towards 275's all around with 17 or 18x10

I run 265/40-17 front and 295/30-18 rear.  With my shocks at a stiff setting I have no problem getting the truck to rotate and I'm just running a V6.  Unless your truck has a much better weight bias than stock running a big tire out back shouldn't be a problem IMHO.  I regularly go into 4-wheel slides or kick the back end around pylons with a bump of the throttle if I'm getting too close. I wouldn't give big rear (street) tires that much credit on a truck.

I noticed more plowing and understeer with softer shock settings but with things stiffened up I feel that it's pretty well balanced. I have somewhere around 60 or 70 events on the wheel/tire setup now (almost one a week for the last 4 years) and I feel pretty confident that it's working.  I don't know what it would be like with slicks or auto-X tires though - that may really change things.   

rentedmule, are you running posi/LSD? i assume that would make a big difference aswell since both tires are trying to move together around corners if you're gassing it

I have the stock G80.  It's better than an open diff but in general it's not great.

hmm, i think i'll start out with an 18x10 and some sticky 275's all around and we'll go from there. sound like a good plan?

Yup.

.... ya know, i have some wheels/tires kinda about that size for sale haha

From what I've read and understand you want to size your tire by the weight of your vehicle. If you are running a 1400lb car then you would be hard pressed to get much heat into a 12" tire. If you are building it with the main purpose of running around the track then I would try and set a target weight and size your tires accordingly. There is more to it but it would give you a better idea of what you need versus what you want.


edit* when it comes to weight that can go either way. My current AR Torq Thrust's 17x8's weigh 50lbs each (with bfg sports). The ZX3r's are 18x10.5's weigh in at 45lbs each and the rear is 18x12.5's and weigh in at 49lbs each (with ra1s).     I'm curious as to what one of the soft 8's with a big tire would come in at.
When choosing your rim diameter figure out how big you want to go on the brakes first. There is only so much rotor you can stuff inside a rim.

Weight is not the only determining factor, it works for a baseline, but from there you need to go out and run it, and measure tire temps the second the run is over, inside and outside, and that will tell you not only if you have enough or too much tire, but if your tire is staying flat on the track.

^ Absolutely, there is a lot to it. Track design, tire compound and on and on. I was just trying to give a good benchmark to start from.

I don't think you'd have any trouble getting heat into any tire that would fit under the front of an s-series.

FWIW a reproduction ZO6 18x10 is 7 or 8 lbs heavier than an OEM version. I put the one repro wheel I have on the passenger rear since that's the one that always spins first LOL! Bottom line is they are nice wheels and can be had cheap.

I think the soft 8s are insanely heavy.

i currently have 17x8's and they grip good, but obviously i'll go wider if i can. Should i build the rear to fit 12's and not worry about it if i end up running 10's? i'd hate to build for 10's and end up needing/wanting a 12, plus we all know this thing will look badass with a foot of tire under it

Yes!!! Over build the permanent stuff, and give your self options for the future.
Also think about this. the more tire surface area you have on the ground, the more spread out your vehicle's weight will be over said area.

For your differential, I recommend that you go the limited slip or ratcheting/ Detroit Locker option if you want to do Autocross, or any tight turning/ braking competition. The LSD will allow you to be on the power a little before a full lock up. The Detroit Locker will only ratchet, or release lock if you are off the gas and turning. Once power is applied, the locker will not release UNLESS you have weak or old springs in the differential.
The choice between these two options, for most users, comes down to application, vehicle setup/ purpose, and driving style.
I've seen broken axles from lockers sticking, or people putting too stiff of springs in their lockers, and taking sharp corners even with narrower tires. One of them was me in my old fairlane. I got the hop and stop, and then a big pop. next thing I knew I had a one wheel wonder with 31 spline axles.

Long story short. It's your rig. Do what you feel is right. You're the person driving it.

I think the soft 8s are insanely heavy.

They are.  I weighed them once, but lost the numbers somewhere.  36 lbs sticks in my head.  I don't want to know how heavy they were when I had run flats on the front.   

Don't forget the zexel diff too, transferes power away from the faster moving tire, so you'll have power on the inside of the corner right up until a milisecond of slip, then it'll transfer back to the outside to keep the inside from slipping.

Don't forget the zexel diff too, transferes power away from the faster moving tire, so you'll have power on the inside of the corner right up until a milisecond of slip, then it'll transfer back to the outside to keep the inside from slipping.

I wish I would have scooped one of those up back when SLP was selling the take-outs dirt cheap back in the day.

They still come up on F-body forums pretty regularly. They need a rebuild, but they're around, and are well priced usually. The ones that do require a full rebuild usually go for about $300 last I saw. The rebuild is far from easy, but if you take your time, it is a doable home garage job.