There are two considerations.

A) Bike fit -- For that bike, a 170/60 is a better tire. Lighter, quicker, better handling, big enough to handle the output of that engine.

B) Rim fit -- If your rim is 4.5" wide, the 170/60 is the right tire. If your rim has been changed to a 5.5" width, then the 180/55 is the right tire for that rim, and you should buy one of those (or switch back to a 4.5" rim and use your free tire).

PhilB

 

Both the ST2 and ST4 came with 5.5x17 rears in 2000. The book suggests the 170 for the 2 and the 180 for the 4. That being said there shouldn't be any ill effects switching from the 170 to the 180, at least I didn't notice. I'm sure the peanut gallery will chime in on this one but that's what opinions are for lol

 

You'll notice easier turn in on the corners if you're a sensitive guy.

I actually went from a 190 down to a 180 on the rear of a new bike back in '97 just to get the quicker steering.
.

 

I went from a 170 to a 180 this year. It's kinda hard to tell if the tire size made any difference as handling improved dramatically just going from worn out tires to a set of fresh new ones. I've noticed nothing detrimental changing sizes.

 

No idea why Ducati specs a 170 for the ST2. Consider the only real difference between the ST2 and ST4, which gets a 180, is the engine. A little more HP and I bet that is the only reason there is a different tire spec. As I have mentioned every other time this question comes up, they did the same with my 2001 900SSie. That bike also had 5.5 inch rim. It came with a 170 Michelin Hi Sport. When it was gone, I replaced it with the new at the time Metzeler Sportec M1. The 180 cross-section Metz looked NARROWER than the 170 I had just removed. Why? Different tire profile and design in part, I imagine, but also consider the 170 is probably better for a 5" rim. When you stretch it across a 5.5" rim, you are going to flatten the profile a bit. This could make it "look" wider. Fitting the 180 on a 5.5" rim maintains the tire manufacturer's intended profile and therefore all the R&D and tire review results you likely spent hours pouring over are also maintained...

Handling difference? Who knows. Someone has recently tried a 180 after using a 170 for a long time and claims he can tell a difference thinking the 170 felt lighter. I think there are too many variables to really say for sure. Ride height adjustment, tire pressure, tire design and profile, how you have the bike loaded, how you ride the bike and probably many more. Either way, I think any difference would be fairly minimal and erased from memory after a ride or two unless you're on a track bike trying to shave fractions of a second off your lap times at a particular track...

180s are generally more available as it is a much more common size and it is the right tire for a 5.5" rim. You can't go wrong with one. You could also use a 170 and live a happy life as well if that is what you choose!

I wanted to add, but got side tracked...

Keep in mind there is more to it than "180 vs. 170". There is also sidewall height. The 180 comes as a 55 and the 170 comes as a 60. This is the ratio of sidewall height to width. 170/60 would have a sidewall height of 102MM and the 180 would have a sidewall height of 99MM. This is where tire manufactures can differ. Where and how do they measure sidewall height? In general, the 170/60 will have a larger diameter than the 180/55. This may have as much of an impact on a said lighter feeling than the width of the tire as it would be similar to raising ride height a little. Less of an effect if it is stretched across a too large rim, but none the less it is there. I still think it's pretty minimal, but for academic purposes, it needs to considered in the discussion.

I used this link toplay with three tire size configs. http://www.gearingcommander.com/ You can see the results in the pic.

 

That gearing commander site is a great site.

Good points, all. And thank you!

 

Keep in mind there is more to it than the numbers, every new tire you try will be slightly different from the next. Ie: metzler 180/55- is slightly different from a dunlop 180/55 which is different from a michelin 180/55.

Wait there's more... in the same brand they can change some between the models too. Tire profile (shape of the tire) can alter between models/brands. This is why I run different tires on my track bikes every year to see what the other tires are like.

If you buy a 180/ it is sized by the mfg. to fit on a bike that came with a 180 from the factory, this does not mean it will duplicate the size and shape it is just their recipe for that tire size. 180 was designed to be the ideal fit on a 5.5 inch rim but you can legitimately run either a 170 or 190 on it. The 170 should make the bike a little easier to turn which may be why Ducati put them on the heaviest bike they sold at the time, then it may have been the slightly lower price too.

If I was going to put tires on my st I would put on 180, why? because it is what I have for trackday take off tires. If I was given a set of 170 would I use them? Without a doubt, There will be little to no downside unless you do a lot of trackdays with the bike and are constantly on the edge of the tire.

4.5" wheel is sized to be optimal with a 160 size but you can run the 170. A 170 would be "best" with a 5" wheel that Ducati does not offer.

Take the free 170 and run it and then next tire change you will know which one you truly prefer. Unless you change brands........ or models.

 

The only difference that I noticed going from a few sets of 170s to a few sets of 180s, now that 170s are getting harder to find, is just the size of the chicken strips.

With 170s, the was no chicken strips on the rear and some small ones up front. About the same both sides even though we don't have a lot of fun left turns on most of the riding that the ST sees.

With the 180s, the chickens strips are about the same size front and rear. Being slightly OCD about evenness of some things, this makes me happier.

I cannot say that I noticed a handling, or any other, difference between the two sizes. New versus worn and profile differences between brands and types is way, way more noticeable.

Have a good one.

 

You probably saw this, but .... here's my recent experience with changing from 170's to 180's

Tire Musings

For the record, I will stay with 180's from now on.

Also note, you've been discussing PR4's tire size. Note that the PR4GT has a stiffer casing... designed for the big 700-1000lb touring bikes. My 170's were always GT because I couldn't find the regular ones. My 180's are not GT's. Bot the GT and non GT perform wonderfully on my 5.5" rim (stock).