No clue what the sweet spot might be, but I'm going to drop from 11 to 7 very soon when I move. The miles were never the issue. Having to ride on the expressway in the dark (45-65 mph traffic) for more than half the ride was. Once I get moved, I'm on 35 mph city streets for at least a year. No more riding dangerous MUPs to bypass the even more dangerous expressway. Thankful for all of that.