Here is what I am thinking:
18 wheels
255/45 front, 8.5 or 9
285/45 (or40?) rear, 9.5 or 10
That's all i got lol has anyone had a similar setup or any advice? Thanks!

 

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KC

 

Do you plan to use the car on road track at all? If yes, you want wider tires on the front .... for a street or drag oriented car, the staggered set-up is fine

255 is a pretty good width for the front; and I would put them on 9" wide wheels .... you can look up the tires on Tire Rack and check the "specs" tab to see the recommended wheel rim width range; you want your wheels to be at least the "measuring width" .... measuring width for the 255's I looked up is 8.5" so the 8.5" is OK but I'd probably go with 9" if the price is not much more

you want tire diameter to be pretty close to the standard 27.2" for these cars ... 255/45-18 is right on target at 27.1"

285 tire should have at least a 10" wide wheel; and 285/45-18 will be too tall, you want 285/40-18 -- overall diameter is 27"

 

THANKS! justa driver, not racing just speeding on the lonely backroads lol! this setup looks good from pics, i dont want low profile. thanks, i checked that out & are within range. was even pondering 17 wheels but think i want 18. we are going to tire shop today! have you driven this setup? mine is currently stock

 

that would be great! going to a show saturday & a meet tonight at a bbq/karaoke joint in the country!
who knows when will get another chance at the brothers car collection

 

no I am running 275/40-19 on 19x9.5" wheels on all 4 corners; my overall diameter is a little taller than stock, which is not a problem; I did recalibrate the speedometer (with a tune) so it would stay accurate

I think the 285/40-18 on rear with 255/45-18 on the front will be fine

I'd just suggest that you don't be tempted by using wheels on the narrow end of the range; stay in the middle or toward the wider end of the wheel width range

 

Standard wheel/tire size for the 2007-09 Shelby GT500 are 18x9.5" diameter w/45mm offset... Tires are 255/45/18" front... 285/40/18" rear... If your preference is running a staggered setup, you can go either with 9" 9.5" or 10" wheel sizes...

If your preference is running a square setup on all 4 corners, then I would recommend going with 275/40/18" tires on either 9.5" size wheels or 10" wheels...

 

You could also run 255/45/18" tires on 8.5" 9" or 9.5" wheel sizes as a square setup on all 4 corners as well...

 

Excellent reference table

 

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KC

 

I've been running a 285/40/18 front for years, with either a 285/40/18 or a 305/40/18 rear tire. This is on OEM Ford 18x9.5 wheels from the 2010 GT500 convertible.
I've also run 275/40/18 fronts, and doing it again now, with 285/40/18 rear. Slight stagger.

For an autocross weekend I also ran a 305/40/18 front with a 305/35/20 rear.

There was also a 275/40/19 front / 305/35/20 rear combo.

And a 275/35/20 front / 305/35/20 rear combo.

And looking to run a 19" combo next, probably 19x10 / 19x11 with a 285/35/19 / 305/35/19 combo.

If you're not gonna autocross or road-course race the car, a 255-275 front with a 285 rear is gonna look and perform great.

Do you have a wheel picked out? Size/width/offset?

 

what size wheels did you run those on, and do you remember the offsets?

 

Factory 2010 GT500 convertible 18x9.5 wheels, lol.
Tires were the drag radials I had on the back.
Expected horribly soft sidewalls to flop over in turns, but they did awesome.

 

 

The only downside to installing 285/40/18" tires on 18x9.5" wheels in the front, is that you may encounter some soft sidewall slop in turns during road course racing or autocross applications, but this is just my personal opinion...

 

That will of course depend on which tire you'd go with. Some tires will surprise you with their sidewall stiffness, like my 305/40 drag radials did.

I ran that size on the front of my car for years with no issues. It fits very well on the 9.5"-wide wheel. A 275 definitely fits more square-shouldered, and I would recommend 275/40/18 as the perfect size for an 18x9.5 wheel, especially if looking to go with the same size all around.
Budget kept me this year from doing the 19" setup I'm looking at next.
I keep hoping I'll win the lottery though.
I may not tell people, but there will definitely be signs. Lol

 

I also wanted to run 285/40/18's in a square setup on my OEM GT500 18x9.5" wheels, but after test fitting them in the front, for some reason they wouldn't clear when turning the front wheels towards full lock which would cause some tire contact at the strut location... I probably could have corrected it by upgrading to the Roush Steering stop kit, but I didn't really want to reduce or limit the amount of full steering lock during turns...

In the end, I compromised by running 275/40/18's in a square setup which just as you brought up most definitely fits more square-shouldered while also providing enough clearance up front when turning the wheels towards full steering lock...

 

I'm running a small space in the front when I have 285/40/18s on there. Just about 5mm. Only to give it a little more clearance against the side of the strut.

 

That's a very good point, Gabe... Perhaps if I decide down the road to Autocross or road race my GT, I just may consider upgrading to 285/40/18's in a square setup by also adding those small 5mm spacers in the front...

The spacers would also make a far better alternative over installing a steering stop kit onto the rack and pinion steering unit... But at least for the time being, the current 275/40/18 square setup has been more than sufficient enough for normal street use applications, at least IMHO anyhow...