Now a question. I run about 20 mph with 48 volts (speedo on the way) but the specs on the controller and motor claim they are good to 60 volts. What do you think?
That's a tough question to answer. Here are a few things that go into it:
1) Motors don't care much about voltage unless we're into the hundreds of volts; that's where insulation starts to break down. Motors DO care about current, though, and higher voltages can push more current through a winding. Too much current is a bad thing; it overheats the motor. Brushless motors are more immune to this than most other motors (there are just so few parts) but with enough heat you can cook the bearings or even damage the magnets.
2) Brushless controllers, alas, do care quite a bit about voltage. Most controllers nowadays use FETs, and FETs have a rating called Vbrdss (voltage breakdown, drain to source) that indicates how much voltage they can "switch against." If the voltage exceeds the Vbrdss of the FET, it will either be damaged or will "avalanche" (turn on all the way, even when it's not supposed to.) This can blow the fuse (if you're lucky) or the FET or even the circuit board (if you're unlucky.)
3) Vbrdss indicates what voltage the FET can withstand, but that does not mean that a 60 volt FET can be used with a 60 volt supply. When phases switch off, a "spike" of voltage is generated due to inductive flyback. This is supposed to be conducted back to power/ground by the body diodes of the FETs, but they are not always fast enough to 'catch' the spike. Thus, an inverter designed to operate at 48 volts is going to use at _least_ 55 volt FETs (if not 60 or 75 volt FETs.)
4) You can't just go up in voltage on the FETs. Typically higher voltage FETs have higher on resistances, which means a lower current rating. If the controller's current limit is not adjusted to a lower rating, this can overheat the FETs and cause failure.
5) Another gotcha - a 48 volt lead-acid system will be charged at 60 volts, and will float around 55 volts. A 36 volt lithium-ion system will be charged at 42 volts and will float at 42 volts. Generally you have to design the controller to handle the maximum voltage the system can see unless you have an interlock to keep the controller off above a certain voltage.
Anyway, if you have a "48 volt system" then the system is likely really designed to operate at 60 volts. If you want to add one more 12 volt battery to bring that up to 60 volts, the controller will really have to deal with 75 volts. (Or 69 volts if you use an interlock.) To see if your controller can handle this (from a purely FET perspective) open it up, get the part # off the FET and look up its Vbrdss.