new speedo and tach problem
 

So I just bought some new gauges for my bike and installed them today. But when I was installing them, I accidentally popped off the needle part of the gauges (the part that actually tells you how fast you're going). So I put them back on with no problem, and started her up. The gauges were completely wrong so I put them back down to Zero where they should be when the bike is off. The problem is, when I put them back to zero something must not have been alligned correctly or theyre completely ****ed, but the speed is off by about 10mph and the tach is off between 500 and 1000rpm's. How can I fix this? Is there a special way I need to go about recalibrating the needles? Thanks.