i wouldent worry about it, its not like your putting some radicly smaller tires on it. your speedo is accually very inacurate anyhow and usually in the slower then real direction so your probably more accurate now. are you worried about miles on the car? i really hope not, its pointless to worry about miles now considering the mods you have done.

I know the offroad truck guys put on swampers and get way slower speedos, they will read 40 when going 60.
you couldnet be changing yours more then 1-2 mph at 60.