want taller wheel and thinner tire...

I have a 94 trans am. it currently has 245/50/16 tires on 16x8 wheels. Is it true that if the height of the tire is decreased then the height of the wheel can be increased and still fit? It the 50 on the tire is 50% ratio of the tire width, then if I decrease to 40%, can I put on a taller wheel. Basically, I am asking if I can put on a 245/40/18 with a 18x8 wheel and still fit my car?