lacpac5

I've searched the web to try and find an option my father 1992 Old 98 regency is claimed to have. Some type of highway package with a larger gas tank and big overdrive gear? I don't think it exists, yet I drove 404 miles on 5/8 tank of gas at 75-78 mph including a 55 minute traffic jam that ate some of that mpg. It took 14.4 gallons from 3/8 of a tank to fill. So that's at least 28 mpg at 75+ mph and a gas tank of 23-25 gallons. So what is this "highway Package" and why can't I find specs on it anywhere on the web? Help?