2240:60 is the ratio you want.
I am not actually sure what the ratio is relative too, i have heard people say its something like "2240 rpm at 60 mph", but I am not sure how correct that is.
A speedo has a magnet inside of a cup thing that is driven by the cable attached to the front wheel. As the cable increases velocity, so does the magnet, and so does the cup thing (I think it may also have a magnet in it as well...). And as they speed up, the magnet on the cup trys to catch the magnet in the cup due to some physics stuff, and there is a hair spring connected to the cup, which is connected to the dial, so the dial is relative to how fast the cup is moving.. I think.
Someone please correct me if i am wrong.