Just took my 2005 S60 2.5T (80,000 miles) on a 250 mile round trip on a flat interstate. Just two people and a couple of bags in the car, and I ran on cruise for most of the time at about 75mph. I've driven the car on trips like this before and done close to 30mpg, but this time I averaged 24 mpg. What gives?

The car is running fine - no obvious issues and I am using premium gas.

What should I look for to explain this lower mpg?