i think the wireless ones get f'd up by the spark plug... maybe? i dunno, mine has been balls on accurate from what i can tell. i measured around the tire with a string, measured in inches, and converted to mm. if its measuring one click every revolution, and multiplying it by the circumference, divided by time.. how can that be wrong? i've worked with GPS with some school stuff before and i know for a fact that it can be off in terms of speed readings. i think the bike speedo is much more sound in theory. plus the one i have is capable of reading up to 99.9 mph, so the processor resolution must be fast enough for accurate readings at 30 mph.
i really really wish i had a back light though, thats really killing me, but what do you want for ten bucks.. seriously? i should just wire a lightbulb to sit above it, or take the face out of a stock puch speedo and put the bike computer in there.