It's a highly debated topic, and people's preferences vary (some like to spin short cranks, some like to mash long ones, etc.). I doubt a 5 mph difference is anything other than placebo, but 170 sounds about right to me. I'm 5'10 with a 31" inseam and I generally ride 170 or 172.5 cranks. In my racing days, I rode 165 or 167.5 on the velodrome and 175 on mountain bikes, so you can see where it depends on what you do and how you do it.