Your in town MPG looks good, that's about what I with my '03. Your highway MPG should be in the ~mid 20s at 75-80...Its a 2007 around 106K..My dad had the plugs changed by Nissan around 90K..I did also just change the air cleaner but didn't look too bad and will look for cracks..
I did a tranny flush last month and just replaced the radiator ( the plastic top had cracked)
What should it get on the hwy running 75-80? Seems to get around 18 in town and last trip got 20 on the hwy. I am heading out again next week and can see if the new air filter and injector cleaner helped.
I was looking online, our walmart has the 2 pack..for $3 might give it a try..I used to add Lucas fuel treatment, the small bottle, in my Mercedes and it got better mileage. I think Walmart sells a knock off that "looks" the same.Funny you should ask, since I was going to post about this... Using STP High Mileage fuel additive, my city MPG went from 16 to 20 over the course of six months. About three months ago, I couldn't find the 2-pack for $3.18, so I stopped using it, and my MPG dropped to 17.1. After finding it again about a month ago, my city MPG climbed to 19.5 in less than a month. My highway is 24.2, going as high as 25.1 if driving very conservatively. I use cruise control the majority of the time, even in city-type environments when I can, so I'm sure that also helps with MPG. GIven the iffy CATS, I'm really surprised MO has done so well. I started using that STP additive around 2015, when my MPG was very low at I think 19/15.