From all my researching and looking around, this seems to be the steepest road in all of Duluth - North 5th Avenue West, between 5th and 6th Streets. It's a full 25.5% grade. It also seems to be one of the worst roads in all of Duluth, it's practically gravel. If your car is older or otherwise weaker, I seriously recommend leaving it at the bottom of the hill and walking up. Especially, especially, especially if it's winter - don't even bother trying if there's snow.
The reward for climbing the road, though? A great view of this beautiful city!
This cache is hidden just off the road on public land, please respect any private landowners. Have fun and stay safe.