Yes, you are chaning the ratio but I'm sure someone with more knowledge will tell you that there is something you can do to the speedo where it hooks up to your transmission. That xcell spreadsheet will tell you exactly what change is made going from 15 -> 17 and different tire size. Do you have it/need it? PM me your email adress if so
Think of it this way. A tire taller (larger diameter) than stock has to turn fewer times to cover the same distance, while a shorter tire has to turn more times. Again, it's all relative to whatever tire size the speedo gear was set up for.
Changing rear end ratio also has the same effect, more or less revolutions to cover the same distance.
GMJim did a real nice chart to help figure out what you'd need, or at least get close.