Riddle me this...

I am turning 3000 rpm at 65 mph. I measured my tire height at 24 inches, although discount tire says 25 online. My rear end is a 7 1/4 marked 2.76 on the outside. Shouldn't I be going a lot faster at that rpm? Could it be that the previous owner swapped gears in the rear end? Could it be the torque converter (it's stock rebuilt)?

Car is a 71 dart with 318 and 904 transmission. The transmission is a "new" rebuild from performance automatic and it shifts good and firm. Fluid in neutral is within the correct range on dipstick.