DaveM and Staklid are spot on. The only observation I'd make is our rear ims are 5.5" wide and the stock tyre is 180/55.
If you look at tyre manufacture fitting charts I think you'll find a 170 will do a 4, 4.5 and 5.0" rim, not a 5.5" inch. The 180 will do 5, 5.5 and 6 etc. I haven't checked this for a while but while a 170 from some manufacturers might fit a 5.5" rim you still need to consider other factors such as the size and shape of the contact when cranked over. The smaller tyre may steer quicker but will also have a smaller contact patch. My old TDM was an 85hp bike, the 170/60 was fine on that, the Sprint is 125hp, 50% more horsepower and a smaller contact patch could be a bit scary
Also with the 180 there is a huge choice in profile amongst the different manufacturers. If you want a quick steering tyre, then try the Avons as folk report they have a much more triangular profile and "fall in" to cornering quicker than some others.
Russ
