for what it's worth the stock 19's that are a 40 series run a 255/40R19, on the brembo cars. It would be worth it for the sidewall sake to run similar.

 

The earlier year V6 base models came with 16 x 7 wheels.

The earlier GT500's came with 18 x 9.5 wheels, with 255/45-18 front and 285/40-18 rear tires. Hard to go wrong with the same sizes on your sixxer as what the top dog Mustang for those years wore.

FWIW, 18 x 10's can be made to work both front and rear, but you have to be very picky about the offset and you may be limited on the tire sizes you can use (potential clearance issues). There aren't as many ways to get this one right than it is with 18 x 9" or 9.5" wide.

The regular GT came with at least one 18 x 8.5" wheel option up through 2008 or so. Don't waste your time with wheels narrower than that, just shop newtakeoff.com, craigslist, whatever, for the OE Bullitt wheels.


Norm

 

The stock 19' wheels on the 2010+ cars sport two tire sizes. On the 19x8.5, Ford puts on 245/45-19. That gives a 27.7" diameter. The track pack cars have 19x9 with 255/40/19. That gives a 27.1" diameter. Some good tires are available in those sizes - including max performance tires. However, I haven't been able to find those sizes in any of the extreme performance tires. I'm running the 245/45-19 on my 19x8.5 rims. (Bridgestone S-04's) Fills the wheel well pretty well. The ride is good and the handling is excellent.

 

18x9.5 with 255 /45 zr18 will be the correct 27" diameter, and 9.5" is the optimum size for a 255. Unless you have crazy power there is no need for more tire on the rear. Lots of tire choices in this popular size, arguably the better handling, and more practicable choice.

 

I'd probably go 265 or 275's on a 9.5-inch rim (if you can find them in your desired brand.

I was shopping autocross tires for 18x9.5-inch wheels, and I was amazed at the lack of variety regarding widths.