I own a 2004 Infiniti G35X sedan with 71K miles. I drive the car 45 to 50 miles per day on the freeway to-and-from work. It has over the past 6 years provided 19 to 20 miles per gallon. Over the last few months the cars gas mileage has dropped to between 14 to 15mpg. The car seems to run the same, and there's been no drop off of performance. I use 89 octane gas...and have the car serviced regularly at the dealership. Any ideas on what's up? Thanks! - Dave -