I just drove over 1000 miles on the freeway average speed about 78. Every time I filled up, i checked my mileage and was under 13 mpg each time. This drive was 500 miles one way and 500 back. I got the same mileage both ways(in other words, it wasn't all uphill or into the wind) I have been getting this type of mileage the past year or so. Before that, I never checked. Is there anything I can do or is this normal?

Chris