At that point you will sacrifice ride quality. Thats why if the car is lowered 18's seem to work fine.

Most people don't realize that the Mustang comes with several different diameter tires, spreading over 1.1" in difference.

V6 with 17 = 28.0"
GT with 18" = 27.2"
GT with 19" A/S = 27.7"
GT with 19" Brembo = 27.2"
Boss 302 rear = 26.9"
GT500 with 20" rears = 27.9"

In this regard, if you had a normal 245/45R19 (27.7") All-Season equipped GT, which is .8" taller than the 285/35/19 (26.9") rears on a Boss 302, and then jumped to to a 20" 285/35R20 rear tire (27.9") would give you .4" shorter sidewall than the stock 19" AS tires, but a 100% equal sidewall height to a Boss 302 rear tire.

Additionally, the 20" tire at 27.9" rear tire will give you a better ride than the 26.9" rears, while giving a little more overall grip from a slighty larger contact patch.

And there you have it. Solid post sir.

18s are definetly noticeably smaller when comparing to 19s and obviously more noticeably with 20s. I myself am tempted to go the 18s route on the Forgestar FR5s I want buy, but in the future want to go big brake as well (brembo at a minimum) and don't want to have to deal with getting different wheels again. I just hate how 19s are such the odd cat with tire selections and 20s are EXPENSIVE!

19s or 20s?
 

Excess rotational weight is horrible for drag racing, but its not always a bad thing for road racing (and even drifting). The extra weight on the axles help keep it planted in the corners. It's on off the reason that help ILE and ZL1 pull more Gs than GT and Bosses. Its not all about the IRS. That extra weight help keep them planted.

For those against the 20" wheel thing, with the 2015 Mustang right around the corner, I would assume that there will be a 20" wheel/tire option available from the onset. Here's a quick history on the tires sizes offered:

1979-84 Mustang = 14" (forget the Michelin TRX metric sizes)
1984-86 Mustang SVO = 16"
1985-90 Mustang GT = 15"
1991-93 Mustang GT = 16"
1994-04 Mustang GT = 16" std, 17" opt
2005-09 Mustang GT = 17" std, 18" opt
2010-14 Mustang GT = 18" std, 19" opt
2015-?? Mustang GT = 19" std, 20" opt

I ride on 19" Brembo's, but 20" wheels in a few years will look pretty normal on a Mustang from the way the trends have gone over the last 35-years.

Weren't you have a lot of traction problems at Ennis?

+1

All the high performance cars use a staggered wheel fitment. Corvettes, Vipers, Ferraris, Porsches etc. Even the GT500 comes stock with 19s front and 20s rear.

As others have stated, wheel weight makes the most difference for unsprung weight. TSWs Nurburgrings are excellent. I saw one member on another forum with a 19X9.5 TSW N that weighed 22lbs. In that size is it definitely lighter than stock AND allow for a larger brake kit. Also on those other cars, because the front fender of those exotics are generally much shorter than the rear fenders, if they ran same size front and rear wheels, the fronts would look bigger.

-Alex

Also keep in mind when you opt for larger wheels, most likely, you will be opting for wider tires. Wider means more tire which means more weight.

-Alex

No traction until 4th gear most runs... And that's if second gear wouldn't lock me out. My car is running Star specs which are more suited for road course/autocross use. Hard sidewall and require a decent amount of heat to grip anywhere, not to mention a 275/35/18 which changes my gearing quite a bit. I'm into 5th gear quite a bit to hit my 113mph trap lol I honestly think I would have been better on the stock all season Pirelli tires.

This thread is all over the place, I run 20x10 and 20x11 wheels and they are about 17lbs lighter per wheel then the stock 19" Brembo wheels. Granted to get that weight loss in a 20" you will have to spend a little money for them. As far as ride quality, I have yet to have a complaint, most everyone comments on how comfortable it is. Suspension setup and the Recaro seats have a lot to do with that as well. Nick/Pepper is right though, if you throw some thin tires on there, the ride is going to suck. As far as looks, pretty sure most the mustangs run with 20" wheels once they go aftermarket. From all the ones that I have seen, they only look bad if the wheel design/finish is lame, or if the fitment is wrong.

Edit: As far as the OP is concerned, I wouldn't even consider 19's, its a waste of money as far as tires go, 18's and 20's give better options for track/show use. Just my 2 cents though, I take my car to road courses as well as shows so I am a tad biased.

Remind me of the wheels you have again? That's an impressive weight!

Forgeline S3C3 (I think thats the right model number). They offer monoblock wheels as well which are even lighter.

Wow they have some really nice wheels! Prices are pretty steep about $1,200 a wheel. Get what you pay for I suppose.