70 degree rise with 140f discharge from the vent. Add 5-10 degrees for the tempature exiting the furnace, you are proably close to hitting the high limit. Is the furnace a standard 80% induced draft/electronic ignition? Increasing blower speed should get you more efficient operation.
Total fan control can be done by installing one of these:
http://www.amazon.com/ICM251-Fan-Con...m_sbs_indust_3