The voltmeter on my 1989 F-150 fluctuates a lot, but most of the time it reads in the area of the gauge.
But everytime I put a voltmeter on the battery terminals the reading is always around 14.5 volts.
The gauge drives me crazy, it never matches the volt meter

 

I have a 88 f150 and it acts the same. I have replaced the alt. and battery and put amp meter in to see what was going on. The voltmeter in the dash moves around a lot , but the amp meter says it is not discharging when the voltage is down to 11volts. The truck runs fine and the battery is staying charged. I have read several posts in here and it seems that in this model year the alt does not put out a lot of curent at low rpms, so if the battery is staying charged don't worry about it.
Mike