Getting less than 35 mpg in my 2nd Gen Prius?

I have an '08 Prius with 275k miles on it. I just reset the dashboard after replacing the inverter cooler pump and I can't even get the average MPG up to 35 mpg after driving 30 miles at speeds up to 60mph with cruise control on. Right now it sits at 33.8 mpg and it took me a good 30 minutes of driving at a steady speed to get it to that point. Any idea what could be wrong?