I'm new to the site and I've been reading a lot of good information. I know there are several other gas mileage threads on here but I can't really find much info relating to my current problem. I bought a 2001 Frontier CC S/C Automatic (which I like very much) last week with 109,000 miles on it. I changed the air filter and went on a trip which was all freeway for about 180 miles and got 12.3mpg. I then filled up (with premium) and drove another 160miles over hilly country roads and got 15.5mpg. I then came back on the freeway for 180 miles and again got about 12-13mpg. Now I know that these trucks are not great on gas but I find it weird I am getting worse on the freeway. I was only running 65-70mph at about 2400-2500 rpms. Any suggestions as to what the problem may be? Knock sensor, O2 sensors, ect.?