I bought a used 2018 Chevy Equinox, LT, front wheel drive about nine months ago. I've averaged around 25.8 miles a gallon driving in town. The last month its dropped to 20.2, then 19.7 and now 17.8. I've had it in for service and the spark plugs were changed and they put something in the fuel line to clean it out. Nothing has helped, it's getting worse. They ran a diagnostic test and everything checks out fine. It is winter now, but the drop started before it got cold. Air in the tires is fine. At a loss and can't understand this. Vehicle just turned 54,000 miles.