Thoughts on tire sizes

[Ugly John]

I'm getting within a couple months of needing to buy a set of tires for autocross. I'm driving a GT PP, and staying in FS, so my wheels will be the factory 19x9 up front with the 19x9.5 in the back. These tires will be dedicated to autocrossing only, with maybe a track day or two thrown in, but no daily driving (I'll be toting the tires to the track on a small trailer). At this point, it is looking like I'll be going with RE71R's unless a better option comes along.

The rear will be 275/35x19, but my question is what should I run for the fronts? The factory 255 doesn't corner as well as I'd like, so I'm thinking about going to either 265 or 275 for them. With the added bulge over the back tires, which would be optimal? I know nothing about sidewall flexing, and I'm not sure if I'll gain more from the wider rubber up front, or if I would lose more from the sidewall bulge. The specs say the 275 will fit on a 9" wheel, but I'm not sure which way to go.

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[Competition Orange]

square

 

[NightmareMoon]

275 up front if you're running a stiff sidewalled tire like the RE71R, especially if you have extra camber up front (as allowed in the FStreet rules)

With a soft sidewalled street tire and factory camber, there might be more reason to respect the rim width and run a 265/255 to avoid a sluggish response and grip lost due to sidewall rollover, but the RE71Rs stiff construction holds up just fine with a 275/35 on a 9" rim.

 

[wildcatgoal]

Can't you do 9.5" up front, too? Or 9" square. I'm not sure you'll need the extra .5" in in the back for autox - I guess it depends on your driving style (if you prefer to purposefully rotate the car with calculated oversteer or a smoother, more clinical approach). The extra grip in the back biases the car toward understeer which I find particularly terrible on autox. Personally I'd want to try grabbing a pair of front GT PP wheels for the back to make a square setup, then do 255 RE-71 (if they make them) all around. Or be sneaky and put the 9.5" wheels with 275s and 5mm or so spacers up front, but put the front wheels in the back with 255s.

 

[Magnificent Bastard]

I'd have to agree with square setup as well, and use the widest rims you can that could still be considered Stock under your class on all corners.

 

[NightmareMoon]

He has to stick with 9" fronts and 9.5" rear wheels to stay in F-Street class.

He has more choice in tire size, including square 275s on those wheels (which allows you to at least rotate the tires on the rims once for longevity). Some guys run 285s square.

 

[Magnificent Bastard]

Fair enough. I think 275 is pushing it on 9" rims for a performance application anyway - 285's would be pretty squirmy on those rims. So that makes sense.

 

[BmacIL]

275 square.

 

[Ugly John]

Thanks guys! It'll be 275's all around. That's what I was thinking, but talking to someone else (non-mustang autocrosser) and he kept bringing up the sidewall flex issue as a possible concern.

And yes, I need to keep on the 19x9F & 19x9.5R to stay in class.

I have new struts sitting around waiting for install, and I'll be slotting them before they go on so I'll have the camber maxxed out.

 

[BmacIL]

It's a bigger issue for road course work than autocross. 275 still is an acceptable fitment for a 9" wheel, from the manufacturers, albeit at the limit of it. It'll be worth having the extra rubber.

 

[Static_LV]

I run 275 RE71s on stock wheel widths in F-Street as well and there isn't any issue with sidewall flex on those tires.

 

[NightmareMoon]

Thanks guys! It'll be 275's all around. That's what I was thinking, but talking to someone else (non-mustang autocrosser) and he kept bringing up the sidewall flex issue as a possible concern.

And yes, I need to keep on the 19x9F & 19x9.5R to stay in class.

I have new struts sitting around waiting for install, and I'll be slotting them before they go on so I'll have the camber maxxed out.

FWIW, I needed 5mm spacers to get enough clearance with 275s on the 9" factory wheels with correctly slotted struts (about 2.4 degrees of front camber is what I got with ~2mm of strut hole slotting). I think mine still rub sometimes too.

 

[Ugly John]

FWIW, I needed 5mm spacers to get enough clearance with 275s on the 9" factory wheels with correctly slotted struts (about 2.4 degrees of front camber is what I got with ~2mm of strut hole slotting). I think mine still rub sometimes too.

Are 5mm spacers allowed in FS using the factory wheels? Or is the requirement to stay within 7mm of the factory offset only applicable to replacement wheels?

 

[kz]

Fair enough. I think 275 is pushing it on 9" rims for a performance application anyway - 285's would be pretty squirmy on those rims. So that makes sense.

Most of the nationally competitive people ran 285 square RE-71R on '15+ Mustangs. Worked fine for them.

 

[kz]

Are 5mm spacers allowed in FS using the factory wheels?

Absolutely.

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