Did the wheels and tires feel that much heavier? Hard to imagine it would make that much difference. Check the air pressure. With the shorter stiffer sidewalls maybe it wouldn't look as low on iar even when it is. Try near max rated pressure and see if that helps.
If the diameter is different from factory, it will register miles wrong. Depending on which way it could show you losing mileage even if you really aren't. Are you making the same drives and still adding gas at the same time? If so maybe it isn't off by so much, just saying it is.
Borrow a GPS and compare the speed it says to what your speedo says.