Well to answer your question there are TWO US Navy Submarine Classes that have "PUBLISHED" maximum speeds of 25+ knots (Approx 28 mph) notice I said published, they will not divulge maximums of any of their equip. These two CLASSES ARE THE VIRGINA CLASS AND THE SEAWOLF CLASS... Listed Below.... If you look further down you will see an small article about a Russian Sub that had top speeds of 78+ knots.
General Characteristics, VIRGINA CLASS
Builder: General Dynamics Electric Boat Division and Northrop Grumman Newport News
Date Deployed: Commissioned 23 October 2004
Propulsion: One nuclear reactor, one shaft
Length: 377 feet (114.8 meters)
Beam: 34 feet (10.4 meters)
Displacement: Approximately 7,800 tons (7,925 metric tons) submerged
Speed: 25+ knots (28+ miles per hour, 46.3+ kph)
Crew: 134: 14 Officers; 120 Enlisted
Armament: Tomahawk missiles, twelve VLS tubes, MK-48 ADCAP torpedoes, four torpedo tubes.
Ships:
USS Virginia (SSN 774), Groton, Conn.
USS Texas (SSN 775), Groton, Conn.
USS Hawaii (SSN 776), Groton, Conn.
North Carolina (SSN 777) - Estimated delivery in December 2007
New Hampshire (SSN 778) - Keel Laid 30 April 2007; estimated delivery in October 2008
New Mexico (SSN 779) - Named 7 December 2004; construction began in January 2004
Unnamed (SSN 780) - Construction began in December 2004
General Characteristics, SEAWOLF CLASS
Builder: General Dynamics Electric Boat Division.
Date Deployed: USS Seawolf commissioned 19 July 1997;
USS Connecticut commissioned 11 December 1998
USS Jimmy Carter commissioned 19 February 2005
Propulsion: One nuclear reactor, one shaft
Length: SSNs 21 and 22: 353 feet (107.6 meters)
SSN 23: 453 feet (138.07 meters)
Beam: 40 feet (12.2 meters)
Displacement: SSNs 21 and 22: 9,138 tons (9,284 metric tons) submerged;
SSN 23 12,158 tons (12,353 metric tons) submerged
Speed: 25+ knots (28+ miles per hour, 46.3+ kph)
Crew: 140: 14 Officers; 126 Enlisted
Armament: Tomahawk missiles, MK-48 torpedoes, eight torpedo tubes.
Ships:
USS Seawolf (SSN 21), Kitsap-Bremerton, Wash.
USS Connecticut (SSN 22), Kitsap-Bremerton, Wash.
USS Jimmy Carter (SSN 23), Kitsap-Bremerton, Wash.
The Russian Alpha class nuclear-powered submarines had a reported maximum speed of over 74 km/h (40 knots), and were believed capable of diving to 760 m (2,500 ft). It is believed only one now remains in service, as a trial boat. Designed by naval architects MG Rusanov and VA Romin, the Alpha class nuclear submarines were constructed at Admiralty Yard in St Petersburg and Severodvinsk. The Russians recently had to take the submarines out of service because of their high maintenance costs