On a "same day" trip, I'd agree with Midpack on the point about wind. On a similar round trip last year... from Leesburg, to Lake City, my 98 Town Car went from 28MPG to less than 20MPG... Rte 75. Same car, same distance, one tank of gas. A very slight wind, but enough to affect the mileage, or at least that's what I figured.
I find the same thing here on my round trip of just 30 miles back and forth to the lake. A 10 mph wind does the job.
My 96SLS and the Town Car are supposed to take Premium, but haven't tasted it since we bought the cars, except for a few test fill ups that didn't prove anything.