a 165/80/15 is gonna be shorter than a 165/15. the second number is the aspect ratio, it denotes the sidewall height, in this case, it's 80% of the section width(165mm). when you see a tire size with only 2 numbers, as in 165/15, that means the aspect ratio is 100% of the section with(the tire is essentially a 165/100/15), which means a taller sidewall, thus a taller tire. 165/15s are about 25 inches tall, 185/15s are about 26.5. your 165/80/15s are only gonna be about 24 inches. 
