Holley's attempt at a nice one-size-fits-all sizing method. (I'm sure it works for some people) Driving with a vacuum gauge hooked to manifold vacuum is the best way to size the PV. You'll see what the vacuum does, when, and then you can pick a size that enriches right when you need it to. Man those plugs sure are black as you move down the porcelain, especially for 25 miles...
