SwayBar

Large diameter wheels will NOT increase relative gear ratios if you choose the proper aspect-ratio tire for a given rim diameter.

There is no reason why you cannot have both; lucky for you, you are a bit mixed up on this gear-ratio thing. One can have a 15 inch rim/tire combination be the exact same installed height as an 18 or 19 inch rim/tire combo. And with those bigger rims, one can install Big Reds or whatever without a problem.

A 15 inch wheel would look absolutely HIDEOUS on a 928 because it's so disproportionately small with respect to the 928's cavernous wheel wells. I've seen a car with the very setup you propose and it literally hurt to look at it.

Easily the single biggest improvement to make a 928 look modern is to ditch the small stock wheels and replace them with at least 18's to help fill-up the wheel wells. Pop GTS mirrors on too, and people think it's a new car from Porsche.

DFWX

I agree that small wheels look awful and the aspect ratio of the tires can offset change to the final tire circumferance. My choice (maybe a bad one if they will not fit with the brakes) was an economic one. What is the price of 18/19 inch wheels in the 12-14 inch width with +2 tires? I do not have that kind of $$ at this time, given all the other thngs money must go for.
Mark 0

SwayBar

Probably the cheapest you can do is a stock twin-turbo rim; I'm not sure, but they're at least 11 inches wide.

I understand the money-thang, I just wanted to encourage you to buy something which is aesthetically pleasing when you look at the car. At least for me, I consider the 'schwing' factor extremely important when I look at any of my cars. Don't buy something you don't like to look at; save up more money if you have to in order to get what you really want since rims/tires are so important to the look of a car, especially a 928.