What size of tires should I put on 18x9 wheels for my 98 GT?

 

im not sure whats the biggest you can fit on your mustang. but looks like you should be fine with 275/35/18 on the back and 245/40/18 on the front. about .1 inch shorter with the 245/40 then the 275/35. so you should be fine with these sizes

 

it's ok to have different sizes on the front/rear?

 

alot of people do it to have that wider tire in the back but alot of people cant put that wide of a tire on the front for rubbing reasons