Just got my GT500 wheels/tires put on. Stock GT500 setup w/285's in the back and 255's in the front. They hook up waaaaay better than my stockers, but my handling seems to have taken a hit. feels way too squishy when changing lanes and making relatively fast turns. My old tires didn't feel that way at all. Is it because I have staggered tire sizes now? Or could something else be wrong? Had them balanced when I got them just in case, so that shouldn't be it.

 

Can you tell if it's the front or the rear that doesn't feel quite as "stuck down"? Squishy all the time? Only when you first start to turn? Later after you're into the turn a bit further? Does it "push" or is it "loose"?

What tire pressures are you running? I don't know what the GT500 pressures are supposed to be, but that car's got quite a bit more weight up front anyway. Tire pressure tuning is a place to start, and with different tire sizes I fully expect that you want different inflation pressures.


Norm

 

If feels squishy all the time. My lane changes at speed take a bit of correcting now, because it feels like the rear end of the car is playing catch-up with the front end. It's a feeling similar to when my panhard bar bushing went out, and the rear axle was literally having to play catch up with the rest of the car. And the front wheels don't feel as stable in harder turns anymore, throughout the whole turn. I'd love some tips on what PSI to use in these tires as well, because I just have 40psi in each as of right now (I'm prepared to be flamed for saying that, since I have no idea what the "correct" pressure is). The way my car feels now could be totally normal, too, since I have no experience with this type of staggered set-up, and the wheels and tires I put on are definitely heavier than my old ones.

 

Hmm, I have the same size/tire setup that I just recently went to and love it. It handles so much better imo. I am running 36 psi in all 4 for now. Its still to cold out here and snowing here and there for me to get much seat time to see what psi works best.

 

I just went to 275/40/18's all the way around on the SVT Cobra (GT500) wheels at 34 psi and she's as stable as ever. Perhaps the tire you have has too much sidewall flex. But at 40 psi I'd think ANY tire wouldn't flex enough to feel the way you describe.

 

I'm not sure what my problem is, then. My D-specs have arrived, so I'll have the shop take a look around underneath to see if there is something wrong with one of my suspension components. I just noticed this afternoon a very slight back and forth shaking of the steering wheel, as well. All of the wheels are put on correctly against the rotors, so maybe I have a front end alignment issue.

 

I'm guessing that it's the40 psi in those rears that's mostly responsible, as I think that much inflation is starting to lift the shoulders and reduce the contact patch. It may be enough even up front to make the car "skate" slightly. If you can't sneak a peak at a GT500 for the sticker-recommended pressures, try starting at 32 all around and go from there. Don't be afraid to run different pressures front vs rear.

Just my opinion, but a 285/40 really belongs on a wider wheel than 9.5", especially in the case of tires intended for daily street use. Ford "cheaped out" a little here, as nobody is going to rotate 255's and 285's front to back anyway (so having different wheel widths would have made exactly zero difference in terms of maintenance).

If your tires are specifically the Eagle F1 Supercars, be aware that their performance in cold or even cool weather has been questioned. Tinkering with inflation pressures will still help you, but can't work miracles when the tread compound decides that it's too cold to play outdoors.


Norm

 

I'll be sure and play around with the psi in the tires, and hopefully that will help. Thanks for the advice!

 

Norm

I will be OK putting 285/40/ZR18 KDW2s on my DD and Track usecar on Steeda 18 x 9.5 rims. I realize the 9.5 is at the minimum side of the BFG recommended sizes of 9.5 to 11 wide but am I going to run into problems? My intent was to have the same size front and back to allow for swapping them front to rear.

 

I don't see any big problems, with the possibility of softish cornering response if you were to run it hard at an auto-X and you're fussy about that sort of thing.If the actual amount of sidewall "bulge" is any indication, it might be similar to the OE 235/55's on 17 x 8's (which aren't all that bad), though you have a greater amount of overall grip to work with. Bottom line, I'd say it'd be a non-issue for most drivers in reasonably normal street driving.


Norm