17.5 racing is very, very dependent on track layout, grip, etc... with what gearing you can get away with. It is hard to provide optimum settings for everything. Going fast with 17.5 takes allot of tweeking

I've seem optimum gearing anywhere from 28/74 all the way up to 33/69.

That being said, start on the lower side of that gearing, and add start checking temps.

Refer back a few pages and I put up a post on how to approach gearing and timing settings for new setups, just to get you used to the philosophy that I use at least.