Every gauge on your stock cluster is going to be ballpark and not completely accurate. That's just how they are from the factory and being 20+ years old does not do anything to help them...
If it bothers you that much you can always pop the cluster out, remove the clear plastic cover, plug it in for a minute to get it to stay put and then pull the needle and move it a little to the left when you put it back on.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.