Why am I getting poor gas mileage at only 8 to 10 mpg with no apparent issues?

Hi this is Dusty, I'm one of the auto experts on RepairPal's Car Genius team! I can definitely understand the concerns about your Navigator's fuel economy, as engines wear, and the odometer rolls fuel economy will decline a bit. It'll vary case by case, but 8-10 mpg isn't far off the 13 mpg (combined) rating when the vehicle was new, and since it runs great it could just be age related. Here's the link if you'd like to check it out: https://www.fueleconomy.gov/feg/bymodel/2006_lincoln_navigator.shtml As for the downshifting when climbing a hill, that's pretty normal, the holding of the lower gear and not wanting to upshift in to the next could be a problem. Even though there's no check engine light, it may be wise to have a diagnosis performed because there's still valuable data that could be useful in pinpointing an intermittent issue that may be causing issues, without raising any flags. If you have any other questions or would like to help with finding a RepairPal Certified shop in your area, give us a call! 877-913-1554