1. It is not normal. You may have destroyed a sensor, or stretched a wire. Something along those lines.
2. Battery may have been dead due to being unhooked for so long. Battery's on it's way out possibly.
3. Speedo will read SLOWER than you are actually traveling. How much slower, depends on your stock tire size and your new tire size. Whatever the percentage difference is, will be the difference. This also effects your odometer too.