I believe that was one of the years that they were using a 20 gal. tank with a long filler hose that would only allow 15 gals into the tank. That was cheaper than making all new tanks for those years. It was actually sold as a 15 gal tank. The previous owner probably found that out, and shortened the filler hose to allow full use of the 20 gal tank. The gauge was meant to show full at 15 gals. So in short, unless it bothers you, I wouldn't worry to much about it. If it does bother you, take a full 5gal tank of gas with you, and run your main tank till it runs out of gas. Notice what the gauge reads at this point, and then you will know when you are out of gas. Now put in the 5 gals of gas, that you have in reserve, to get to a gas station.