18's with 275s on a Duster

If i cant measure the wheel width, its cause its got a tire on it, and i can measure the tire. An 18" section width tire with a 3 1/2 bs will fit, with 5 1/2 it wont. Dont know, or care, what the offset is.
Edit: curious why it is"ET" and not something self explanatory, like "offset ", or "off" or a simple + or - number?