Here's the deal. My gas mileage has been decreasing by around .5 mpg each fill up (once a week) since the middle of August. It was averaging about 23 mpg during the summer with either a/c or wide open windows, but at my last fill up it was only 19.87 mpg's. Is something wrong mechanically since it's been a consistent downspiral -- the mpg hasn't gone up a single time? Halfway through this trend I filled my tires, but that certainly didn't make a difference. I don't think I'm driving more aggressively with every fill up -- yeah, I normally keep my car in the slightly higher rpm's, but I've kind of always been doing that. I wouldn't be concerned if it was seesawing -- up one week, down the next, because that would be understandable. However, a slow but steady dive is kind of scaring me. Any ideas?

 

Get a tune up?