It could be worse
You're lucky it's reading low. My '89 MK VII came to an abrupt stop several hundred miles from home, the gas gague was reading 1/4 full. I had it towed to the local rural Ford dealer, rented a car from him to get home (it was late Thursday), only to have him call me the following Monday to tell me the bad news: I had run out of gas. Fortunately the service manager had a family function the next weekend and offered to drive it not too far from my place. I took it to the dealer I bought it through (I got it through the Ford vendor purchase plan). They replaced the sender, for free and Ford sent me a check covering the cost of the rental! Needless to say I was astonished. I forget the term that was used, but it had something to do with customer inconvenience with a luxury car. Those really were the days, eh? I can't believe such a thing would exist today.
Now my "off topic" question: anyone fit a Whipple or Kenne Bell type supercharger under the hood of an '89 style Mark? My GT40 motor runs like a bat out of hell, with the latest horsepower ratings comming out, I want to remain competitive in my "old man's car."