My '98 frontier ka24de gas mileage has been slipping just a bit. For a couple months I consistently averaged about 26 mpg. For the last few weeks it's been closer to 24mpg. I drive the same route to work each day, about 30 miles each way. I don't run the a/c a lot. I've filled at the same gas station for the most part, with a few attempts at variety to see if it made any difference. I usually run 87 shell. My last tank was 93 octane, mpg was 24.08. Any ideas or suggestions?