on 6s im geared to 53mph which is nice. Great torque, great speed... 5s may lower the top end but should still be fun. I wouldnt gear for too much speed as the esc may get hot..
I plan on running my 1717 with the stock 49T spur and a 25T pinion. See how I like that on 6s. If its not enough, I may spring for the 43T spur.