I have low water pressure in my house, which is supplied by a well system. It seemed to have happened over a months time, but is now to a point where it is difficult to take a shower.

The pressure gauge was bad, so I replaced it, and the on/off presures seem to be good. The pump goes on around the 40 mark, and off just before the gauge reads 60 psi. Pressures seem to hold well according to the gauge, until water is used. But the faucet has very little pressure, and the shower on the second floor barely works.

Could the pressure tank be the problem? What is the best way to find out what the pressure currently is in the tank? Will an automobile tire gauge work?
Any ideas would greatly be appreciated. I'm hoping it's not the well pump.

The pressure of the system is determined by the amount of pressure in the holding tank. Most tanks in the last 20 years or so are pressurized. They have a rubber bladder in them that holds a certain pressure in it above the water. If that low side of the pressure switch is 40 psi, the bladder pressure should be 38psi. To check this, you must drain the tank completely then check the pressure in the tank. You can use a tire pressure gauge for this. If it is under that 38psi, use a small compressor (like a tire pump or small electric compressor and bring it up to the 38 psi and NO HIGHER!!
Let the tank sit for at least one hour then recheck the pressure. If it is low again, the bladder is ruptured and the tank will need to be replaced.
What happens in storage tanks is, the bladder keeps the water in the tank fro absorbing the air and keeps a cushion of pressurized air to constantly have a down pressure on the water.

Do you have a house filter or water softener? If so, change the filter, and put the water softener on bypass and let me know if things are better. If the pressure tank is bad, or water logged, you will notice the pressure switch contacts opening and closing over and over again in a short period. Please let us know what you find.