It has to do with moment of inertia and it's not just that the 19" combo is heavier. It's that it's heavier farther away from the center of the wheel. As weight moves farther from the center it has a exponentially worse effect both on acceleration on and on stopping.

Think of a lever and the saying "give me a lever long enough and I will move the world"..

Here, for braniacs (unlike myself) this site should give headache.. lol
http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

For people more like myself..
http://en.wikipedia.org/wiki/Moment_of_inertia
&
http://www.britannica.com/eb/article-9042379

Ok, so they are talking about bike wheels.. same principal..
http://www.canecreek.com/168.html

So to sum it up:
Polar moment of inertia
The resistance of an object to rotational acceleration. When the mass of an object is distributed far from its axis of rotation, the object is said to have a high polar moment of inertia. When the mass distribution is close to the axis of rotation, it has a low polar moment of inertia

Wheel weight is bad.. wheel weight farther from the axis of rotation is worse. That why a 20-inch wheel that weights 30lbs is far worse in regards to acceleration and braking when compared to a 17" wheel.. even if they both weighed the same (which I doubt they would) since most of the weight of the 20" combo is so far from the center point of the wheel.

Did any of that make sense? lol

It's not that you shouldn't run 19's or 20's or whatever. It's just that you should know the physics behind what you decide to do and understand what trade offs some decisions entail.

If you do the math all the tire side walls are about the same
335/30/18 is 26.1 tall and 13.3 on a 12 rim pilot sport
345/30/19 is 27.4 tall and 13.6 on a 12 pilot sport
335/30/20 is 28 tall and 13.5 on a 12 pilot sport 2
these are all Michelin tires
Pirelli has 345/25/20 @ 26.89 and is 13.43 on a 12.5 rim
and 355/25/19 @ 26.02 and 14.06 on a 13 rim
So it all depends on how you want it to look with the 20 you can tuck the rim up into the fender and the car is at the same height as having most of the tire showing with the 18" also if you already have a gear set for the rear then the tire size makes a differance as well. ......... oh yea the cost also the 18 are a little cheaper but not much

Mmmm....
 

Let me muddy the water just a bit. Stock on a C6 vette is 18's in the front and 19's in the rear. With run flats and a z-51 suspension get .95 on the skid pad. Run flats are pretty hard tire. C6 Z06 are running the same 18/19 combo but I believe it is a 275 up front and a 305 in the rear...it pulled right at 1.0 g.

I understand the physics of rotational enertia, even if I can't spell it, but on a street car I am not sure you would loose that much, but I have no practical expeirence with this. I will tell you to run as much sidewall as possible, you will thank me the first time you hit a pot hole you did not see. I have 18's on an 850 and I can tell you I spend most of my time looking for holes while I am driving.

I believe the hot set-up for track cars is an 18 in front and 17 in the rear

very good piont payton...sometimes I think people don't realize these are STREET cars....the majority aren't going to notice the difference in handling in a 19 verse 18 anyway

Im going to go out on a limb and say that there may be other influences involved in the 1.0g than tire size.

True, BUT that doesn't mean you shouldn't know what's going on. Making INFORMED decisions is what this is all about.

The worse thing in the world is for someone to slap on huge wheels and think it's the same as the smaller deal, just bigger.

18x12 335-30-18 pilots 68 convert looks perfect!!!!!!!

I might have four of those for sale.. lol

Ill take all 4 we go through alot of tires :D ( burn outs)

got the 19by12 19by10 combo on my 69 there are photos on a post somewhere