Suggest to read the user manual thoroughly to understand the basic idea of design rules and net classes. Routing grid is just another parameter which has to be set in a trade-off between ideal gridless routing and processing time. Track width and clearance is controlled by design rules.
More generally speaking, don't expect too much from an auto router, particularly from a simple one like the Eagle built-in. Using an auto router for a design that you don't manage to route manually is usually in vain.