Gauge

So, I still have a few bugs to work out but the one that bugs me most is my speedo. It is off between 5-7mph.I wasnt armed for it but I went to pull the gear to see how many teeth it had, in case that was the culprit. I wasnt ready for the flow of fluid that came from behind it (retard moment). So, I just showed it back in and left it be. Anyway, as I drive away, the speedo will stay pegged at 0 until I get up to some speed, and then all of a sudden jump up and do its thing. It seems to stay a constant 5-7mph off no matter the speed I go. I also, notice that it gets sporatic around the 45 mark. But, I kind of blame the front left wheel bearing that yells at me every time I drive it. Is it likely, that I need to dig in and figure out me speedo gear size? Could it be electrical or mechanical w/in the dash? The jeep originally had 235/55 17s on it . But I wanted to wait til I got new tires (got 235/75 15s now)before I knocked out the speedo issue.