After 10 posts you aren't a newb anymore and you do have a rock crawlin' avatar
Seriously though, xxx/xx/16 and xxx/xx/18 can be the same size tire except one goes on a .......wait.......wait.....16 in wheel and the other goes on a ......wait......wait.....18 in wheel. If you don't know the difference, that's fine - but, if you don't know the difference there is probably no reason to get anything other than something that fits a stock size wheel. Maybe upsize the tire for looks or future off road, but leave the wheel alone (sorry Loud).
People like to go down to a 16 because they can get an increase in sidewall. I like the 18's personally and there are more and more tires coming out that fit the wheel and give you an increase in sidewall. Again, if you don't need it or want it, why bother?
If you want to know the tire size take the first number and divide by 25.4- then multiply by the second number as a decimal to get your sidewall. Multiply the sidewall by 2 and finally add the third number.
275/60/18
275/25.4 = 10.82 x .60 = 6.5(rounded) x 2 = 13 + 18 = 31 inch tire
That's my Karma post for the day.