The first number is the width of the tire in mm, and the second number is the height of the tire sidewall referenced as a percentage of the width so a 170/80 is 170 mm wide and 80% of 170 = 136mm tall while a 180/70 is 180 mm wide and 70% of 180 = 126mm tall. My guess is this tire would fit but I'm no expert. In this case, you won't have a problem with the tire fitting under the fender (like the front) but you'd have to make sure it doesn't rub on the sides (swing arm and chain or chain guard)due to the extra width.
The tires that fit with the stock front fender are either 100/90 or 110/80 which give you sidewalls of 88 and 90 mm respectively. A 120/90 would be 108mm and 130/70, 91mm. I don't know how much extra space the Honda fender gives you but you'd need about 3/4" over stock to get the 120/90 to fit. Of course you may have width issues as well with these wider tires.
Again, I'm no expert, but IMO, Triumph spends a lot of time figuring out what size tires will work best on their bikes. I'm guessing you will not feel much of a difference between a 170/80 and 180/70 on the rear. I would not run a 130/70 on the front (if it did fit which it may not)as I believe this will make the steering very slow and heavy.
If a bike is so large, heavy, or powerful that it needs a wider tire (like the Rocket III) or if you are just going for a "style" like the custom chopper guys- fine, otherwise the bike will probably handle better with the narrower rubber.
Keith
