I agree, that you must have more gear than you think or some really short tires. I am running 4.88's with a 31 inch tire and turn 3000 at 55. I think I am getting better milage than you, and I have a 4500 stall and a heck of a cam in the motor.
373's should be a great gear for the street, good performance and decent milage. You just might have a tranny problem.