Everybody,
I can tell you why you're getting terrible gas mileage. The computer system for the RL recognizes what type of fuel you use. It does this to control the knock and ping from your engine. Meaning if you use anything below 91 octane, the computer will retard your timing. Anything above 91, it runs like it was designed.
How do I know, I tested this approach when I first bought my 96 with 99k miles, and the Repair manual discusses this.
When I bought 89 octane - I got 14.2 mpg
When I bought 93 octane - I get (today) between 19.7 - 20.6 mpg
Thanks for the reply! However, I religiously put Amoco Gold (93 octane) in my car because I was aware of the anti-knock control. I guess something is wrong with my cars computer. Thanks