JoeyD said:
the 18" wheels were going to be Konig Reasons 18X7.55-100/11445GRM
and the tires were going to be Kumho Ecsta AST 225/50R-16 92H BSW
JoeyD:
Your tires won't fit on your rims, you need an 18" tire for an 18" rim! Also don't put 17" in front and 18" in the rear, the mkiii is so front heavy already you will be shifting the front to rear center of gravity even further forward, not good for handling, unless you want to look like an 80's Camaro.
Gelboy:
When you look for tires try to find something that has close to the same overall diameter as the stock size 225/50/16 are around 24.7"-25.1"
The 225/50/18 is around 26.9-27.2" it will look like a SUV tire in the Supra's wheel well. If you wanted to stay with the same width as stock (although I don't know why if you are buying all new wheels and tires), you would want something like a 225/40/18 (overall dia. 25.1").
You could up size the rims width if you haven't bought anything yet to 8.5-9" wide in the front and 9-10 wide in the rear with the correct offset these fit just fine (10's may need the fender rolled). Then run 245-255 in the front and 265-285 in the rear.
If you already own the 7.5" wide rims then a 235-245 wide tire is about all you can fit on there. I've put 245/45/16 on the stock sawblades, but it was bubble walls galore, didn't look good at all.
Having said all of this, you need to read, read, read. There a bunch of very informative threads on how to choose wheels and tires on this forum, SF, mkiiitech, and many other forums out there.