My bugeye speedometer reads about 15% high as well, but the odometer is reasonably close to the correct value. So, before changing gears, you might want to check the odometer and if it's OK, the gearing is OK. There is probably a way to tweak the speedometer internals to get it closer, but I haven't dug into mine yet. Back in the 60s, most speedometers erred a few percent on the high side, so to some degree this isn't unexpected.