Fixed rates are well below 5% today. Even my tiny local banks, that are usually higher than big banks, are posting 4.75.
I'm sitting at 5.875 and if I've run it right I'll save about 115/month going to 4.75. Assuming that's all interest I could add that 115 to principal and be paid off 6 years early without a change in today's required payment. Since I already send in an extra 50/month, 165 extra gets me paid off 8 years early without changing what I actually send in now.
Hrm.