Mitsun,
As you pointed our there are calculations for determing speed loss/gain with different tire/wheel sizes. For someone in your position who completely swaps outwheels andgoes much smaller thenI'd leave the speedo alone for the stock tires/wheels and just figure out what speed the othertire/wheel size gets you and drive accordingly. If you are the artistic type you could always make a stick-on speedometer readoutthat covers your existing one, but is calibrated for the smaller wheels .

In your situation it would be nice to have the GPS speed, but I don't know of too many folks that swap out wheels here in the Mid-Atlantic states.