The gauge reads 12v with the key on in the start/acc position and jumps to 15v while the car is running. Is this normal? I thought it was just supposed to read 12v all the time
Should be around 13.8 with the car running. Chech voltage at the battery with a multimeter, if you actually have 15volts, chances are your voltage regulator is bad.
I have my needle just touching the 15 volt tick as well (it's actually at 14.6 volts). As noted, just pull the gauge and put your DMM on the power and ground to see what the actual value is.