I have a 92 500E. I drive about 70% on highways (avg 80mph) and 30% around town in terms of miles. Recently, I'm finding that I'm getting around 13-14 mpg, which seems lower than I used to get. I live in Florida, so cold weather isn't a big issue. When I lived in Nevada, I was getting around 16-17 mpg with similar driving habits.

Any ideas on why it would decrease?