(1) Locate the ride distance along the bottom axis
(2) Go up to the percent of ride that is level (or proportion climbing and descending). You may need to interpolate.
(3) Read off the ride grade in the right-descending diagonal box.
(4) Look up the recommended level speed for the ride grade here. The grades marked in red as "Walk" mean a rider of this grade would need to walk up the steepest hills.
