i was just wondeing whats the diffeence between regular tires and run flats. right now the elky has cooper cobras on it and some rims but im not sure of the size. i was thinking about getting a new set of rims for it maybe something like 18s all around or 18s in the front and 20s in the back but was wondering whats the diference in tires

Run-flats are expensive and less performance oriented. Carry a can of "Run-Flat"; much cheaper.