bad milage

So I bought this 2.3 2wd long bed ranger over a year ago with the intent of putting a lot of highway miles on it. I spend the winter swapping the transmission to a 5 speed and now that its on the road this summer i’m noticing it gets really bad gas milage. I get about 18 mpg in the city and 18...

2.3L 4speed Auto with over 200k and no extra weight besides an aluminum tool box getting 18MPG. When I bought it used a few months ago, I figured it just had old parts and I started to do some tune-ups. Replaced spark plugs and wires New air filter Cleaned MAF Changed Oil and filter Cleaned...