factory tires on 17's are 32.05" The average 305/70 r 16 or 33 r 17's are 32.5". The big difference is the section and tread width. You can check out the different tire companies web sites for specs on the tires. A 1/2" of increased diameter is hardly a reason to re-gear. However, the increased drag of the extra rubber against the ground and possibly the more rugged tread may make a gear swap seem more appealing. My spedo was off by about a mile an hour or so at 65mph. The dealership was able to flash the computer for the new tire size.

If your using your factory wheels, check out the section width of your stock tire vs. the width of your new tire. You should divided that in half and put that big of a wheel spacer behind your wheel.